Add annotations about collections-extended

[mlenzen]

Not going to do, invalid syntax for old versions.

[mlenzen]

Maybe only support recent versions in 1.0

[mlenzen]

Can support 2 & 3 by putting the annotations in a stub file or as a comment

• https://www.python.org/dev/peps/pep-0484/#stub-files
• https://www.python.org/dev/peps/pep-0484/#suggested-syntax-for-python-2-7-and-straddling-code

[mlenzen]

Check that autodoc uses type annotations https://pypi.org/project/sphinx-autodoc-typehints/

[mlenzen]

Need to create a unique type for each Sentinel so that we can add that type to the annotation.

[mlenzen]

Need to use backported typing library for Python <3.6

[mlenzen]

Can move annotations out of stubs now.

[jzazo]

Can setlists and rest of types be annotated? When I am using this module mypy tells me Type of variable becomes "Any" due to an unfollowed import. It would be nice to have annotations. Thank you.