Introduce Y022/Y023/Y024/Y025: Imports linting error codes

[AlexWaygood]

Helps work towards #80 and #32. I've tried to keep these error codes completely cross-compatible between Python 2 and Python 3 -- I think it would be best to leave Python-3-only changes (e.g. disallowing typing.ContextManager and typing_extensions.OrderedDict) can be left for another error code.

[AlexWaygood]

Okay, should be ready for review now. I'll hold off running a codemodding script on the Python-2 parts of typeshed until people have had a chance to review :)

[AlexWaygood]

I will get codemodding tomorrow!

[AlexWaygood]

Typeshed PR to make the CI go green:

• Use PEP 585 syntax in Python 2, protobuf & _ast stubs, where possible python/typeshed#6949

TODO:

• Try to refactor the dicts, per @Akuli's request
• Disallow importing Type from typing_extensions as well as typing
• more codemodding, apparently! Use PEP 585 syntax in @python2/_ast, convert more TypeVars to _typeshed.Self, & # noqa a SQLAlchemy line python/typeshed#6954

[AlexWaygood]

Ohhhh I hardcoded Y019 to look for type[_T] (but not Type[_T]), so changing all occurences of Type to type in Python-2 typeshed stubs has caused the CI to pick up on a bunch of new Y019 errors.

[AlexWaygood]

flake8 is complaining that PyiVisitor._check_import_or_attribute is too complex for one function. However, I think it would make the code more complex to split it up into several smaller functions, so I am going to # noqa it. Let me know if you disagree.

[AlexWaygood]

flake8 is complaining that PyiVisitor._check_import_or_attribute is too complex for one function. However, I think it would make the code more complex to split it up into several smaller functions, so I am going to # noqa it. Let me know if you disagree.

Actually, I'll leave it for now, since I might be refactoring that function anyway.

[AlexWaygood]

A second typeshed PR to try to get the CI green is here: python/typeshed#6954

[AlexWaygood]

...And a third typeshed PR: python/typeshed#6955

[AlexWaygood]

CI is finally green, and I think I've addressed the points @Akuli raised (just two dicts now, and managed to get rid of the icky quotes-within-quotes everywhere). Re-requesting review from you, @JelleZijlstra, if that's okay, as it's changed a fair bit since when you approved it :)

[Akuli]

Skimmed. No red flags.

[AlexWaygood]

Skimmed. No red flags.

Thanks!