Add Python version specifiers to [core] dependencies

[mgorny]

Summary of changes

Add Python version specifiers to importlib_metadata, importlib_resources and tomli dependencies, to require them only on Python versions on which they are actually used.

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[jaraco]

Unfortunately, applying this change will almost certainly break the update routine for the vendored packages due to pip not respecting environment markers. If we add these exclusions, we'll also at the very least need to add them to the workaround in that routine. Or perhaps something similar to the upgrade_core could be done to strip the environment markers as a workaround for direct dependencies.

Even better would be to find a solution to pypa/pip#12770; perhaps uv doesn't have that issue and could be used instead.

[jaraco]

In 022cedb, I switched to uv for vendoring and it works great, so no longer have any concerns about the markers.

[mgorny]

Thanks!