WIP: allow sphinx-autodoc-typehints to be upgraded to 2.3.x on Python 3.9+

[mr-c]

Summary of changes

Allow installing with sphinx-autodoc-typehints version up to version 2.3.x in Python 3.9+ systems.

Currently this results in an error while building docs, hence this PR being in draft state, see #2912

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[ashleysommer]

After RDFLib v7.1.0 releases, we will drop Python 3.8 support, and from RDFLib v8.0 we have only Python 3.9+.