Omit chunks with no elements in slice selection with step (zarr-devel…

…opers#1154)

* Omit chunks with no elements in slice selection with step

This stops chunks being read unnecessarily when a slice selection with a
step was used. Previously all chunks spanning the start-end range would
be used regardless of whether they contained any elements.

Fixes zarr-developers#843.

* Test that only the required chunks are accessed during basic selections

This tests that only the expected set of chunks are accessed during
basic slice selection operations for both reads and writes to an array.

* Update release notes.

* Fix typos in release notes.

2.13.2: another fix for the conda-forge tests (zarr-developers#1146)

See description in zarr-developers#1143
Testing locally, however, does not suffice. In this case, the relative path
in the test worked for me, but doesn't work on conda-forge. The only reliable
way to be sure is to run `./build-locally.py` in the zarr-feedstock
**against pre-tagged code**.

 2.13.1: generate fixture/meta in conda tests (zarr-developers#1143)

* 2.13.1: generate fixture/meta in conda tests

Following the pattern from zarr-developers#824, this generates the
fixture test if it does not exist so that tests will
pass on conda-forge.

see: conda-forge/zarr-feedstock#65

* Fix rst link

* Fix rst link again

* Add pragma: no cover

Prepare 2.13.0 release (zarr-developers#1133)

* Prepare 2.13.0 release

* Update release instructions for pre-releases

* Add zarr-developers#1142 for Greg's work

* Make python-version match for pre-commits

2.13.0a2 release notes (zarr-developers#1126)

* 2.13.0a2 release notes

* release note cleanup

* Make left-panel links darker

* Try rgb color

* Darken toc text and lighten background

* More light purple

* Even MORE purple!