flake8-pyi's Y027: use collections.abc in pyi files

[twoertwein]

flake8-pyi encourages using collections.abc imports in pyi files.

[Dr-Irv]

I'm reluctantly approving this. Given that many things like Sequence, Mapping, etc. were deprecated from typing in python 3.9, it makes sense to do this. However, I don't fully get why things like Callable and Hashable are part of collections.abc. So now if we add some typing somewhere, we have to remember which ones are in collections.abc versus typing.

[twoertwein]

It is not ideal since we still have to use the old imports in py files. At least for pyi files, flake8-pyi will emit a warning when we import it from the "wrong" place.