Fix imports of typing_extensions

[Dreamsorcerer]

Fixes #5107

[derlih]

@Dreamsorcerer
may be it is better to move all of this kind of importing

if sys.version_info >= (3, 8):
    from typing import Final
else:
    from typing_extensions import Final

to typedefs.py and then import it from there. What do you think?

[Dreamsorcerer]

to typedefs.py and then import it from there. What do you think?

I'm not overly opposed to it, but I think it could make maintenance slightly more difficult. e.g. When we drop support for Python 3.7, I can currently grep for "3, 8" or "typing_extensions" to find every place that should be updated. I don't really want to leave the code importing from typedefs when it's no longer needed.

[derlih]

Then it could be still present in typedefs.py but without version checking.
I mean to find all occurrences is not a big problem. Just run the tests or mypy.

[Dreamsorcerer]

Yep, OK, we'll do it that way.

[lgtm-com]

This pull request introduces 1 alert when merging ff09e43 into 726f44e - view on LGTM.com

new alerts:

• 1 for Unused import