Remove `python2` extra from `setup.py`

[sobolevn]

Refs https://github.com/python/mypy/issues/12237

[github-actions[bot]]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[github-actions[bot]]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[hauntsaninja]

I'm not sure about this... If we keep the extra around, users will get a much more helpful error at type check time, rather than a sort of mysterious install time failure.

My vote would be to clean this up a couple of releases from now.

[sobolevn]

Ok, fair enough!