Drop support for Python 2

[AlexWaygood]

Closes #7367

[github-actions]

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

[ManuelZ]

If I'm using a MyPy version that still supports Python 2, will it be able to download stubs from Typeshed without issues?

[hauntsaninja]

mypy vendors a copy of typeshed. All currently released versions of mypy both a) support Python 2, b) vendor a copy of typeshed that supports Python 2. The next release of mypy will do neither.

mypy does let you use your own copy of typeshed using the --custom-typeshed-dir flag. Very few people use this, but in case this is what you were asking about, the answer is you won't be able to type check Python 2 code, but type checking Python 3 code should still work.

[ManuelZ]

Thanks.