Python 2 feature freeze (and removal of support)

[JukkaL]

Since Python 2 reached its EOL Jan 1 2020 (over two years ago), I think that it's time to not actively improve Python 2 support any more.

Details:

1. We won't accept new PRs that improve Python 2 support, unless they fix a significant regression.
2. We won't drop or degrade existing support for Python 2, for now. Python 2 will become unsupported at some point, but at least not until the second half of 2022 (details TBD).
3. Existing open Python 2 PRs can still be merged if they seem useful and low risk.
4. We can remove some Python 2 specific test cases if they are hard to maintain (e.g. require runtime access to Python 2).

cc @sobolevn who has done some recent work on Python 2 features.

[hauntsaninja]

#12202 was a pretty significant Python 2 regression and we've only gotten the one report from it :-)

[JelleZijlstra]

Worth noting that mypy appears to be the last type checker to still support Python 2. Pyre and pyright never supported it; pytype and pyanalyze dropped support last year. We also stopped updating the typing backport for Python 2. Once mypy drops Python 2 support, we should also remove the Python 2 stubs from typeshed.

[hauntsaninja]

Is Python 2 support still important for Dropbox?

[JukkaL]

Is Python 2 support still important for Dropbox?

No.

[JukkaL]

Worth noting that mypy appears to be the last type checker to still support Python 2.

I think that PyCharm still supports Python 2.

There are also quite a few downloads of the typing backport for Python 2 (on the order of 150k-200k downloads per day): https://pypistats.org/packages/typing