Cleanup after we drop support for Python 3.7

[Pierre-Sassoulas]

When we do drop support because 3.7 has reached its end of life after 2023-06-27, we'll need to do some cleanup:

• Remove the code for _ExtendAction (Misc cleanup after dropping Python 3.7 #8793)
• Import Final, Literal, and TypedDict from typing directly
• Remove functools.cached_property import guards

[cdce8p]

EOL for 3.7 is scheduled for 2023-06-27.

Once 3.7 is dropped, the most notable changes for us are probably

• We can start using assignment expressions
• Final, Literal, TypedDict can be imported from typing directly
• We no longer need import guards for functools.cached_property

[Pierre-Sassoulas]

We also need to upgrade functional tests that are exclusive to python 3.7 so they use py-version instead.