The motivation for this is that typed-ast is a pain to deal with (it's just broken again on latest Python 3.10, and was a pain to deal with for Python 3.9, e.g. preventing Windows installs for some months) and Python 2 usage is ever dwindling*. Since mypy 0.900 will basically force all users to revisit how they install mypy, it seems like a good time to force the remaining users of mypy with Python 2 to change to pip install 'mypy[python2]'.

@JukkaL thoughts?

*pypistats.org tells me Python 2 usage is down 40% in the last six months (based on Python 2 downloads of six)