BREAKING CHANGE: eliminate rdflib.term.Genid and rdflib.term.RDFLibGenid

[aucampia]

Summary of changes

• Added docstring to Graph.de_skolemize method
• Added a new test to test when a uriref parameter is passed to the de_skolemize method
• Fixes eliminate rdflib.term.Genid and rdflib.term.RDFLibGenid  #1828

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.

[coveralls]

coverage: 90.314% (+0.04%) from 90.276%
when pulling de6a8b2 on aucampia:aucampia/20230619T2345-remove_genid
into e8f61d4 on RDFLib:main.

[edmondchuc]

Requires a follow-up from me to fix the conflicts in main.