Use PEP 585 syntax in @python2/_ast, convert more TypeVars to _typeshed.Self, & # noqa a SQLAlchemy line - #6954

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

Does this definitely work? I.e. why did we skip Python 2 when we did this codemod previously?

#5872 just says "scripts did something weird", but looks like we switched Python 2 to use PEP 604 syntax in #5915 and nothing went wrong :-)

Does this definitely work? I.e. why did we skip Python 2 when we did this codemod previously?

From my perspective, it was because I didn't care about Python 2 stubs, and there would be fewer problems to fix after the script had run if I only ran the script on the Python-3-only stub :)

I wrote "e.g. because scripts did something weird", and this particular reason isn't why I skipped @python2 stubs. I didn't touch @python2 simply because I don't care about what happens to Python 2 stubs.

Also, AlexWaygood's scripts tend to be of much better quality than my quick and dirty scripts, so they're less likely to ruin stub files and require manual work :)

Another note: flake8-pyi uses typeshed's .flake8 file in its CI, so configuring flake8 to silence @python2 errors there is also an option.