ENH: rewrite backports from Python std lib in a fashion that can be automatically cleaned up with pyupgrade

[neutrinoceros]

PR Summary

Title.
With the latest version of pyupgrade (that we already use as part of our pre-commit hooks),
"versioned code branches" attached to specific versions of Python 3 will now be cleaned up automatically, making their technical debt effectively 0, but they have to include an else statement.

see asottile/pyupgrade#530

[neutrinoceros]

didn't expect this to break. Will fix later

[neutrinoceros]

This won't work, the implementation we have for cached_property is not a back port of functools.cached_property and is not even compatible with it. Fixing this is not worth the required time for it atm, imo.

[matthewturk]

If I am reading it correctly, I added cached property myself. Should we just change its name to avoid ambiguity with stdlib?

[neutrinoceros]

That would be a good idea IMO