Update typing_extensions imports for Python 3.9

[cdce8p]

No description provided.

[hauntsaninja]

Looks good (although in my own code I've started moving to always importing everything from typing_extensions. This also makes runtime typing more robust)

[cdce8p]

Looks good (although in my own code I've started moving to always importing everything from typing_extensions. This also makes runtime typing more robust)

True, you have to know exactly which typing versions in the stdlib have issues. Like collections.abc.Callable in 3.9.0 I mentioned in another PR. However, I do think the benefit of using stdlib imports is that you can faster check which version of typing_extensions is required for used features. E.g. if I see from typing_extensions import TypeVar, I'd have to use 4.12.0+ for TypeVar defaults.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅