replace pkg_resource with importlib{_,.}{metadata,resources}

[kba]

setuptools 82 has finally removed pkg_resources outright, so the pkg_resources calls and runtime setuptools dependency needs to go now.

This PR:

• replaces pkg_resources with the importlib.metadata and importlib.resources APIs.
• adds the PyPI-provided importlib backports as requirements for older Python versions
• completes Replace deprecated pkg_resources usage in mongo observer #943

Workaround if this PR needs more time is to pin setuptools < 82 in requirements.txt.

[thequilo]

I'm all for removing pkg_resources if it doesn't break on old (but still supported) Python versions, which it currently does (see tests). I'm also fine with removing Python 3.8 from the tests and adding python versions up to 3.14 to the tests.

[kba]

I'm all for removing pkg_resources if it doesn't break on old (but still supported) Python versions, which it currently does (see tests). I'm also fine with removing Python 3.8 from the tests and adding python versions up to 3.14 to the tests.

I have changed the version switches, so the importlib_metadata is used instead of importlib.metadata for Python 3.10 and older did not have the importlib.metadata.package_distributions method.

I have not changed the versions tested because I don't fully understand how this tox workflow works. I would appreciate retaining support for 3.8 while that is possible without breaking compatibility with newer versions because we have some setups for training that still use it.