Fix doc build with sphinx8

[danigm]

Fix #2936

Summary of changes

Adds apidocs/rdflib.namespace to the list of autodoc_skip_member_handler to avoid issues when generating docs because sphinx tries to str(obj) to check if the class .startswith('typing.Annotated').

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[ashleysommer]

This is no longer required, proper fix was in #2964 (and backported to 7.x branch in #2965)