Bump pytest-checkdocs to >= 2.14 to resolve deprecation warnings

[Avasam]

Unblocks many downstream CIs failing due to deprecation warnings.

pytest-checkdocs is also now fully typed and marked as such, but I don't see any linked issue downstream that was depending on that.

[Avasam]

Maybe not necessary if you just want to rely on the current pin (which technically still match the version). But some cache-busting somewhere will still be needed for pip/tox.

This is just solving a deprecation warning, but since warnings are treated as errors with the default skeleton configs, it does make it so any version under 2.13 won't pass. So the bump isn't an "optional improvement".

But also if for whatever reason a downstream project can't update to pytest-checkdocs 2.14, the deprecation warning could be suppressed.

So idk. Still opening just in case.