Fix 939 return cg from parse

[mielvds]

Summary of changes

Fixes #939, which was really easy to do since most of the logic moved out of the CG parse method. Not sure whether this needed additional discussion or not; it does change the public API

Checklist

• [x] Checked that there aren't other open pull requests for the same change.
• [x] 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.
  • [x] Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • [x] Added or updated tests that fail without the change.
  • [x] Updated relevant documentation to avoid inaccuracies.
  • [x] Considered adding additional documentation.
• [x] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 91.018% (+0.01%) from 91.008% when pulling 0ed790921871a440411959d00a85e53ca4cfabe1 on mielvds:fix-939-return-cg-from-parse into a8f37ccea6e70b2156c6434a129f6141bb0d6409 on RDFLib:main.

[aucampia]

pre-commit.ci autofix

[ashleysommer]

Note, with impending removal of ConjuntiveGraph, I don't think this is necessary.