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

• 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: 91.018% (+0.01%) from 91.008%
when pulling 0ed7909 on mielvds:fix-939-return-cg-from-parse
into a8f37cc on RDFLib:main.

[aucampia]

pre-commit.ci autofix

[aucampia]

@mielvds thanks for this, this is a breaking change, so I will hold off a little with merging it, as I want to release 7.0.1 or 7.1 in the next weeks, but I will merge it soon afterwards.