To resolve #888, this should also be added to stdlib/2/typing.pyi.

Looks like adding import sys to stdlib/2/typing.pyi causes

stdlib/2/types.pyi:16: error: Cannot determine type of 'long'

Should I ignore it or remove version check from stdlib/2? Is it actually needed there? Python docs mention it was added in 3.5.3. I think it could be clarified in CONTRIBUTING file.

That makes more sense. :) There are comments in that stub that convinced me that ClassVar shouldn't be defined on Python <3.5.

# TODO: Several types should only be defined if sys.python_version >= (3, 5):
# Awaitable, AsyncIterator, AsyncIterable, Coroutine, Collection, ContextManager.
# See https: //github.com/python/typeshed/issues/655 for why this is not easy.

# TODO: ContextManager (only if contextlib.AbstractContextManager exists)

https://github.com/python/typeshed/blob/master/CONTRIBUTING.md#stub-versioning doesn't help either.

That comment refers to types that are defined in the language or in other parts of the stdlib, and only appear in newer Python versions. But ClassVar is unconditionally defined in typing.py, so it will work whenever a new enough typing.py is present. (True, it won't exist in the stdlib version for Python 3.5.0--3.5.2, but that's where that section in CONTRIBUTING.md applies -- it does exist in the stdlib for 3.5.3.)

Anyway, this still requires changes to mypy as well -- we're tracking that in python/mypy#2771.