Add compat modules for tomllib and importlib.metadata

[chrysle]

The conditional backports increased, thus I added two compatibility modules.

Contributor checklist

• Included tests for the changes.
• PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[WhyNotHugo]

Can you rebase this? The CI failures seem unrelated.

[chrysle]

The CI failures seem unrelated.

They are, but tests are also failing on main. We're struggling to fix them for quite a while now. #2106, which still awaits reponse, partly succeeds in doing that, but then there're still the broken Windows tests and I hate Windows feel unable to reproduce the failure.