Update vendored python-dotenv to fix doubles backslashes when loading .env file

To preface, my apologies if this is also fixed in the current master as @techalchemy noted on some recent posts I made.

* Check the existing issues   
  * Did so, no results for 'backslash' '\\' and the like
* Check the [diagnose documentation](https://docs.pipenv.org/diagnose/) for common issues  
   * Doesn't seem to exist?

### Issue description

When using a `.env` file, pipenv doubles the backslashes but only in the value side of an `envar=value` expression

### Expected result

That the value of environment vars as seen within `pipenv shell` and within python (via `os.environ`) are unchanged from the `.env` file

### Actual result
`.env` with test cases:  
``` 
PIPENV_TEST=IS_THIS_SLASH\DOUBLED
IS_THIS_SLASH\DOUBLED=PIPENV_TEST 
```

Console:
```
:) project $ pipenv shell
Loading .env environment variables…
Launching subshell in virtual environment…
:) path $  . /home/user/.local/share/virtualenvs/project-HNPoG7zF/bin/activate
(ds-ops-tools) :) path/project $ printenv PIPENV_TEST
IS_THIS_SLASH\\DOUBLED
(project) :) path/project $ printenv IS_THIS_SLASH\\DOUBLED
PIPENV_TEST
(project) :) path/project $ python
Python 3.6.8 (default, Mar 27 2019, 13:32:53)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-23)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.environ['PIPENV_TEST']
'IS_THIS_SLASH\\\\DOUBLED'
>>> os.environ['IS_THIS_SLASH\\DOUBLED']
'PIPENV_TEST'
>>>
```


### Steps to replicate

Create a `.env` file containing the above, invoke a virtual env using it and follow the commands above

-------------------------------------------------------------------------------

<details><summary>$ pipenv --support</summary>

Pipenv version: `'2018.11.26'`

Pipenv location: `'/usr/local/lib/python3.6/site-packages/pipenv'`

Python location: `'/usr/local/bin/python3.6'`

Python installations found:

  - `3.7.2`: `/usr/local/bin/python3.7m`
  - `3.7.2`: `/usr/local/bin/python3.7`
  - `3.6.8`: `/usr/local/bin/python3.6`
  - `3.6.8`: `/usr/local/bin/python3.6m`
  - `2.6.6`: `/usr/bin/python`
  - `2.6.6`: `/usr/bin/python2.6`

PEP 508 Information:

```
{'implementation_name': 'cpython',
 'implementation_version': '3.6.8',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '2.6.32-754.10.1.el6.x86_64',
 'platform_system': 'Linux',
 'platform_version': '#1 SMP Thu Nov 29 15:44:01 EST 2018',
 'python_full_version': '3.6.8',
 'python_version': '3.6',
 'sys_platform': 'linux'}
```

System environment variables:

  - `SSH_AGENT_PID`
  - `HOSTNAME`
  - `SELINUX_ROLE_REQUESTED`
  - `GPG_AGENT_INFO`
  - `TERM`
  - `SHELL`
  - `HISTSIZE`
  - `SSH_CLIENT`
  - `SELINUX_USE_CURRENT_RANGE`
  - `QTDIR`
  - `QTINC`
  - `SSH_TTY`
  - `KC_SCRIPTS`
  - `HISTFILESIZE`
  - `USER`
  - `LS_COLORS`
  - `PIPENV_DOTENV_LOCATION`
  - `SSH_AUTH_SOCK`
  - `MAIL`
  - `>`
  - `PWD`
  - `LANG`
  - `PS1`
  - `SELINUX_LEVEL_REQUESTED`
  - `HISTCONTROL`
  - `PS2`
  - `GPG_TTY`
  - `SHLVL`
  - `HOME`
  - `LOGNAME`
  - `QTLIB`
  - `CVS_RSH`
  - `SSH_CONNECTION`
  - `LESSOPEN`
  - `G_BROKEN_FILENAMES`
  - `HISTTIMEFORMAT`
  - `_`
  - `OLDPWD`
  - `PIP_DISABLE_PIP_VERSION_CHECK`
  - `PYTHONDONTWRITEBYTECODE`
  - `PIP_SHIMS_BASE_MODULE`
  - `PIP_PYTHON_>`
  - `PYTHONFINDER_IGNORE_UNSUPPORTED`

Pipenv–specific environment variables:

 - `PIPENV_DOTENV_LOCATION`: `/sandbox/prod/ms_source/scripts/project/config/.env`

Debug–specific environment variables:

  - `>`: `/usr/lib64/qt-3.3/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/opt/dell/srvadmin/bin:/home/user/bin`
  - `SHELL`: `/bin/bash`
  - `LANG`: `en_US.UTF-8`
  - `PWD`: `/sandbox/prod/ms_source/scripts/project`


---------------------------

Contents of `Pipfile` ('/sandbox/prod/ms_source/scripts/project/Pipfile'):

```toml
[[source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true

[dev-packages]

[packages]
atomicwrites = ">=1.3.0"
attrs = ">=19.1.0"
certifi = ">=2018.11.29"
chardet = ">=3.0.4"
colorama = ">=0.4.1"
idna = ">=2.8"
lxml = ">=4.3.1"
more-itertools = ">=6.0.0"
pluggy = ">=0.9.0"
py = ">=1.8.0"
pyodbc = ">=4.0.26"
pytest = ">=4.3.0"
requests = ">=2.21.0"
six = ">=1.12.0"
uptime = ">=3.0.1"
urllib3 = ">=1.24.1"
docopt = "*"
sqlalchemy = "*"
pandas = "*"
xlrd = "*"

[requires]
python_version = "3.6"

```


Contents of `Pipfile.lock` ('/sandbox/prod/ms_source/scripts/project/Pipfile.lock'):

```json
{
    "_meta": {
        "hash": {
            "sha256": "ba75f875d81b10d6d80407d9bb4269ac04eabc8efa82600d9b0c135fce80710d"
        },
        "pipfile-spec": 6,
        "requires": {
            "python_version": "3.6"
        },
        "sources": [
            {
                "name": "pypi",
                "url": "https://pypi.org/simple",
                "verify_ssl": true
            }
        ]
    },
    "default": {
        "atomicwrites": {
            "hashes": [
                "sha256:03472c30eb2c5d1ba9227e4c2ca66ab8287fbfbbda3888aa93dc2e28fc6811b4",
                "sha256:75a9445bac02d8d058d5e1fe689654ba5a6556a1dfd8ce6ec55a0ed79866cfa6"
            ],
            "index": "pypi",
            "version": "==1.3.0"
        },
        "attrs": {
            "hashes": [
                "sha256:69c0dbf2ed392de1cb5ec704444b08a5ef81680a61cb899dc08127123af36a79",
                "sha256:f0b870f674851ecbfbbbd364d6b5cbdff9dcedbc7f3f5e18a6891057f21fe399"
            ],
            "index": "pypi",
            "version": "==19.1.0"
        },
        "certifi": {
            "hashes": [
                "sha256:59b7658e26ca9c7339e00f8f4636cdfe59d34fa37b9b04f6f9e9926b3cece1a5",
                "sha256:b26104d6835d1f5e49452a26eb2ff87fe7090b89dfcaee5ea2212697e1e1d7ae"
            ],
            "index": "pypi",
            "version": "==2019.3.9"
        },
        "chardet": {
            "hashes": [
                "sha256:84ab92ed1c4d4f16916e05906b6b75a6c0fb5db821cc65e70cbd64a3e2a5eaae",
                "sha256:fc323ffcaeaed0e0a02bf4d117757b98aed530d9ed4531e3e15460124c106691"
            ],
            "index": "pypi",
            "version": "==3.0.4"
        },
        "colorama": {
            "hashes": [
                "sha256:05eed71e2e327246ad6b38c540c4a3117230b19679b875190486ddd2d721422d",
                "sha256:f8ac84de7840f5b9c4e3347b3c1eaa50f7e49c2b07596221daec5edaabbd7c48"
            ],
            "index": "pypi",
            "version": "==0.4.1"
        },
        "docopt": {
            "hashes": [
                "sha256:49b3a825280bd66b3aa83585ef59c4a8c82f2c8a522dbe754a8bc8d08c85c491"
            ],
            "index": "pypi",
            "version": "==0.6.2"
        },
        "idna": {
            "hashes": [
                "sha256:c357b3f628cf53ae2c4c05627ecc484553142ca23264e593d327bcde5e9c3407",
                "sha256:ea8b7f6188e6fa117537c3df7da9fc686d485087abf6ac197f9c46432f7e4a3c"
            ],
            "index": "pypi",
            "version": "==2.8"
        },
        "lxml": {
            "hashes": [
                "sha256:03984196d00670b2ab14ae0ea83d5cc0cfa4f5a42558afa9ab5fa745995328f5",
                "sha256:0815b0c9f897468de6a386dc15917a0becf48cc92425613aa8bbfc7f0f82951f",
                "sha256:175f3825f075cf02d15099eb52658457cf0ff103dcf11512b5d2583e1d40f58b",
                "sha256:30e14c62d88d1e01a26936ecd1c6e784d4afc9aa002bba4321c5897937112616",
                "sha256:3210da6f36cf4b835ff1be853962b22cc354d506f493b67a4303c88bbb40d57b",
                "sha256:40f60819fbd5bad6e191ba1329bfafa09ab7f3f174b3d034d413ef5266963294",
                "sha256:43b26a865a61549919f8a42e094dfdb62847113cf776d84bd6b60e4e3fc20ea3",
                "sha256:4a03dd682f8e35a10234904e0b9508d705ff98cf962c5851ed052e9340df3d90",
                "sha256:62f382cddf3d2e52cf266e161aa522d54fd624b8cc567bc18f573d9d50d40e8e",
                "sha256:7b98f0325be8450da70aa4a796c4f06852949fe031878b4aa1d6c417a412f314",
                "sha256:846a0739e595871041385d86d12af4b6999f921359b38affb99cdd6b54219a8f",
                "sha256:a3080470559938a09a5d0ec558c005282e99ac77bf8211fb7b9a5c66390acd8d",
                "sha256:ad841b78a476623955da270ab8d207c3c694aa5eba71f4792f65926dc46c6ee8",
                "sha256:afdd75d9735e44c639ffd6258ce04a2de3b208f148072c02478162d0944d9da3",
                "sha256:b4fbf9b552faff54742bcd0791ab1da5863363fb19047e68f6592be1ac2dab33",
                "sha256:b90c4e32d6ec089d3fa3518436bdf5ce4d902a0787dbd9bb09f37afe8b994317",
                "sha256:b91cfe4438c741aeff662d413fd2808ac901cc6229c838236840d11de4586d63",
                "sha256:bdb0593a42070b0a5f138b79b872289ee73c8e25b3f0bea6564e795b55b6bcdd",
                "sha256:c4e4bca2bb68ce22320297dfa1a7bf070a5b20bcbaec4ee023f83d2f6e76496f",
                "sha256:cec4ab14af9eae8501be3266ff50c3c2aecc017ba1e86c160209bb4f0423df6a",
                "sha256:e83b4b2bf029f5104bc1227dbb7bf5ace6fd8fabaebffcd4f8106fafc69fc45f",
                "sha256:e995b3734a46d41ae60b6097f7c51ba9958648c6d1e0935b7e0ee446ee4abe22",
                "sha256:f679d93dec7f7210575c85379a31322df4c46496f184ef650d3aba1484b38a2d",
                "sha256:fd213bb5166e46974f113c8228daaef1732abc47cb561ce9c4c8eaed4bd3b09b",
                "sha256:fdcb57b906dbc1f80666e6290e794ab8fb959a2e17aa5aee1758a85d1da4533f",
                "sha256:ff424b01d090ffe1947ec7432b07f536912e0300458f9a7f48ea217dd8362b86"
            ],
            "index": "pypi",
            "version": "==4.3.3"
        },
        "more-itertools": {
            "hashes": [
                "sha256:2112d2ca570bb7c3e53ea1a35cd5df42bb0fd10c45f0fb97178679c3c03d64c7",
                "sha256:c3e4748ba1aad8dba30a4886b0b1a2004f9a863837b8654e7059eebf727afa5a"
            ],
            "index": "pypi",
            "version": "==7.0.0"
        },
        "numpy": {
            "hashes": [
                "sha256:0e2eed77804b2a6a88741f8fcac02c5499bba3953ec9c71e8b217fad4912c56c",
                "sha256:1c666f04553ef70fda54adf097dbae7080645435fc273e2397f26bbf1d127bbb",
                "sha256:1f46532afa7b2903bfb1b79becca2954c0a04389d19e03dc73f06b039048ac40",
                "sha256:315fa1b1dfc16ae0f03f8fd1c55f23fd15368710f641d570236f3d78af55e340",
                "sha256:3d5fcea4f5ed40c3280791d54da3ad2ecf896f4c87c877b113576b8280c59441",
                "sha256:48241759b99d60aba63b0e590332c600fc4b46ad597c9b0a53f350b871ef0634",
                "sha256:4b4f2924b36d857cf302aec369caac61e43500c17eeef0d7baacad1084c0ee84",
                "sha256:54fe3b7ed9e7eb928bbc4318f954d133851865f062fa4bbb02ef8940bc67b5d2",
                "sha256:5a8f021c70e6206c317974c93eaaf9bc2b56295b6b1cacccf88846e44a1f33fc",
                "sha256:754a6be26d938e6ca91942804eb209307b73f806a1721176278a6038869a1686",
                "sha256:771147e654e8b95eea1293174a94f34e2e77d5729ad44aefb62fbf8a79747a15",
                "sha256:78a6f89da87eeb48014ec652a65c4ffde370c036d780a995edaeb121d3625621",
                "sha256:7fde5c2a3a682a9e101e61d97696687ebdba47637611378b4127fe7e47fdf2bf",
                "sha256:80d99399c97f646e873dd8ce87c38cfdbb668956bbc39bc1e6cac4b515bba2a0",
                "sha256:88a72c1e45a0ae24d1f249a529d9f71fe82e6fa6a3fd61414b829396ec585900",
                "sha256:a4f4460877a16ac73302a9c077ca545498d9fe64e6a81398d8e1a67e4695e3df",
                "sha256:a61255a765b3ac73ee4b110b28fccfbf758c985677f526c2b4b39c48cc4b509d",
                "sha256:ab4896a8c910b9a04c0142871d8800c76c8a2e5ff44763513e1dd9d9631ce897",
                "sha256:abbd6b1c2ef6199f4b7ca9f818eb6b31f17b73a6110aadc4e4298c3f00fab24e",
                "sha256:b16d88da290334e33ea992c56492326ea3b06233a00a1855414360b77ca72f26",
                "sha256:b78a1defedb0e8f6ae1eb55fa6ac74ab42acc4569c3a2eacc2a407ee5d42ebcb",
                "sha256:cfef82c43b8b29ca436560d51b2251d5117818a8d1fb74a8384a83c096745dad",
                "sha256:d160e57731fcdec2beda807ebcabf39823c47e9409485b5a3a1db3a8c6ce763e"
            ],
            "version": "==1.16.3"
        },
        "pandas": {
            "hashes": [
                "sha256:071e42b89b57baa17031af8c6b6bbd2e9a5c68c595bc6bf9adabd7a9ed125d3b",
                "sha256:17450e25ae69e2e6b303817bdf26b2cd57f69595d8550a77c308be0cd0fd58fa",
                "sha256:17916d818592c9ec891cbef2e90f98cc85e0f1e89ed0924c9b5220dc3209c846",
                "sha256:2538f099ab0e9f9c9d09bbcd94b47fd889bad06dc7ae96b1ed583f1dc1a7a822",
                "sha256:366f30710172cb45a6b4f43b66c220653b1ea50303fbbd94e50571637ffb9167",
                "sha256:42e5ad741a0d09232efbc7fc648226ed93306551772fc8aecc6dce9f0e676794",
                "sha256:4e718e7f395ba5bfe8b6f6aaf2ff1c65a09bb77a36af6394621434e7cc813204",
                "sha256:4f919f409c433577a501e023943e582c57355d50a724c589e78bc1d551a535a2",
                "sha256:4fe0d7e6438212e839fc5010c78b822664f1a824c0d263fd858f44131d9166e2",
                "sha256:5149a6db3e74f23dc3f5a216c2c9ae2e12920aa2d4a5b77e44e5b804a5f93248",
                "sha256:627594338d6dd995cfc0bacd8e654cd9e1252d2a7c959449228df6740d737eb8",
                "sha256:83c702615052f2a0a7fb1dd289726e29ec87a27272d775cb77affe749cca28f8",
                "sha256:8c872f7fdf3018b7891e1e3e86c55b190e6c5cee70cab771e8f246c855001296",
                "sha256:90f116086063934afd51e61a802a943826d2aac572b2f7d55caaac51c13db5b5",
                "sha256:a3352bacac12e1fc646213b998bce586f965c9d431773d9e91db27c7c48a1f7d",
                "sha256:bcdd06007cca02d51350f96debe51331dec429ac8f93930a43eb8fb5639e3eb5",
                "sha256:c1bd07ebc15285535f61ddd8c0c75d0d6293e80e1ee6d9a8d73f3f36954342d0",
                "sha256:c9a4b7c55115eb278c19aa14b34fcf5920c8fe7797a09b7b053ddd6195ea89b3",
                "sha256:cc8fc0c7a8d5951dc738f1c1447f71c43734244453616f32b8aa0ef6013a5dfb",
                "sha256:d7b460bc316064540ce0c41c1438c416a40746fd8a4fb2999668bf18f3c4acf1"
            ],
            "index": "pypi",
            "version": "==0.24.2"
        },
        "pluggy": {
            "hashes": [
                "sha256:25a1bc1d148c9a640211872b4ff859878d422bccb59c9965e04eed468a0aa180",
                "sha256:964cedd2b27c492fbf0b7f58b3284a09cf7f99b0f715941fb24a439b3af1bd1a"
            ],
            "index": "pypi",
            "version": "==0.11.0"
        },
        "py": {
            "hashes": [
                "sha256:64f65755aee5b381cea27766a3a147c3f15b9b6b9ac88676de66ba2ae36793fa",
                "sha256:dc639b046a6e2cff5bbe40194ad65936d6ba360b52b3c3fe1d08a82dd50b5e53"
            ],
            "index": "pypi",
            "version": "==1.8.0"
        },
        "pyodbc": {
            "hashes": [
                "sha256:0e89294da8bd1d1dd37799d07ee9c4ecfea821a9a6e6f51566a0efef5cff0495",
                "sha256:19c2132f7a1c573eae1b5ea8915143243b2b557ea2fae551d833ecbac79b2c6e",
                "sha256:2c673a6d3247c1e2a636d18f3e02742381778c4194da5e583f03dc3ea050598a",
                "sha256:3d00077859d214915c089624c459ec05b823ffc89071657eec109389909b19d3",
                "sha256:44430048ede2b47cb2b1d45f9d6d00d2575752942160f8c75a2a588ae3602e66",
                "sha256:59623016dd429ea2e258a8d4d08b661ae497aeb6ea24e14cb67af3bf3d263ce3",
                "sha256:9d5549c49846a40a0f6e1cd9fd2ab147d4c77452ce74452a103e2c244e38a441",
                "sha256:a229ae7b487a123e2a1d09ad529ce3a7857957c87679246e87874697cad5d9ed",
                "sha256:a3f82f853ae71f5a2be7cdd29e51d197984147d3682fb326eff3e44ea03d617b",
                "sha256:b5e46d768ab7a4ac7621ba1499678e77d2bf143222c60a9c4c3c8f8e7a0c7ef6",
                "sha256:c0747d54c066acf79a4e042906f558a27755939f673479ce4dcd88cb9bab09c9",
                "sha256:d10754f5534dffd04f5ebb2fc321ac59ee6d2f3ddfeb161701094f4e2df63a7e",
                "sha256:e455a58b807ed9c4c7ea3f712bf0ad04a3400baf93a14b016e81219a0eb0831b",
                "sha256:e52700b5d24a846483b5ab80acd9153f8e593999c9184ffea11596288fb33de3",
                "sha256:f1abc331975f1a2131d235f87a683b55c7d55a198fcf5fae02267a26d849e323",
                "sha256:fc661323bd8339759fafb2081e9d20b576c988994f9f8b3dd936f5bc8b974dd8"
            ],
            "index": "pypi",
            "version": "==4.0.26"
        },
        "pytest": {
            "hashes": [
                "sha256:1a8aa4fa958f8f451ac5441f3ac130d9fc86ea38780dd2715e6d5c5882700b24",
                "sha256:b8bf138592384bd4e87338cb0f256bf5f615398a649d4bd83915f0e4047a5ca6"
            ],
            "index": "pypi",
            "version": "==4.5.0"
        },
        "python-dateutil": {
            "hashes": [
                "sha256:7e6584c74aeed623791615e26efd690f29817a27c73085b78e4bad02493df2fb",
                "sha256:c89805f6f4d64db21ed966fda138f8a5ed7a4fdbc1a8ee329ce1b74e3c74da9e"
            ],
            "version": "==2.8.0"
        },
        "pytz": {
            "hashes": [
                "sha256:303879e36b721603cc54604edcac9d20401bdbe31e1e4fdee5b9f98d5d31dfda",
                "sha256:d747dd3d23d77ef44c6a3526e274af6efeb0a6f1afd5a69ba4d5be4098c8e141"
            ],
            "version": "==2019.1"
        },
        "requests": {
            "hashes": [
                "sha256:11e007a8a2aa0323f5a921e9e6a2d7e4e67d9877e85773fba9ba6419025cbeb4",
                "sha256:9cf5292fcd0f598c671cfc1e0d7d1a7f13bb8085e9a590f48c010551dc6c4b31"
            ],
            "index": "pypi",
            "version": "==2.22.0"
        },
        "six": {
            "hashes": [
                "sha256:3350809f0555b11f552448330d0b52d5f24c91a322ea4a15ef22629740f3761c",
                "sha256:d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73"
            ],
            "index": "pypi",
            "version": "==1.12.0"
        },
        "sqlalchemy": {
            "hashes": [
                "sha256:91c54ca8345008fceaec987e10924bf07dcab36c442925357e5a467b36a38319"
            ],
            "index": "pypi",
            "version": "==1.3.3"
        },
        "uptime": {
            "hashes": [
                "sha256:7c300254775b807ce46e3dcbcda30aa3b9a204b9c57a7ac1e79ee6dbe3942973"
            ],
            "index": "pypi",
            "version": "==3.0.1"
        },
        "urllib3": {
            "hashes": [
                "sha256:a53063d8b9210a7bdec15e7b272776b9d42b2fd6816401a0d43006ad2f9902db",
                "sha256:d363e3607d8de0c220d31950a8f38b18d5ba7c0830facd71a1c6b1036b7ce06c"
            ],
            "index": "pypi",
            "version": "==1.25.2"
        },
        "wcwidth": {
            "hashes": [
                "sha256:3df37372226d6e63e1b1e1eda15c594bca98a22d33a23832a90998faa96bc65e",
                "sha256:f4ebe71925af7b40a864553f761ed559b43544f8f71746c2d756c7fe788ade7c"
            ],
            "version": "==0.1.7"
        },
        "xlrd": {
            "hashes": [
                "sha256:546eb36cee8db40c3eaa46c351e67ffee6eeb5fa2650b71bc4c758a29a1b29b2",
                "sha256:e551fb498759fa3a5384a94ccd4c3c02eb7c00ea424426e212ac0c57be9dfbde"
            ],
            "index": "pypi",
            "version": "==1.2.0"
        }
    },
    "develop": {}
}

```
</details>


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Update vendored python-dotenv to fix doubles backslashes when loading .env file #3757

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Update vendored python-dotenv to fix doubles backslashes when loading .env file #3757

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