DEP: drop importlib_metadata as a dependency on Python 3.12 and newer

[neutrinoceros]

This is a follow up to #1260: according to the comment linked in that PR's commit message, standard library importlib.metadata isn't problematic starting from Python 3.12

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[braingram]

Thanks for the PR. I'll try to give this a test with weldx sometime soon.

Unfortunately the downstream testing in the CI will be insufficient to test this change (the possibly affected package, weldx, is being run with python 3.10).

[braingram]

I set up a local python 3.12 environment, installed the source branch for this PR and weldx (I had to remove the upper pin they have for the python version). Running the script in the previous issue succeeded and I confirmed that in python 3.12 we no longer see duplicate entry points for the standard library importlib:

(EntryPoint(name='weldx', value='weldx.asdf.extension:get_extensions', group='asdf.extensions'),
 EntryPoint(name='asdf', value='asdf._core._integration:get_extensions', group='asdf.extensions'))

Thanks!