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.

[ashleysommer]

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

[nicholascar]

Happy to approve this but for the RDFLib 8.0.0 release (Sept 2024??) in since the major version number change will be appropriate for the API change.

We will need to remove the ConjunctiveGraph handling though as CG will be removed in 8.0.0, so we will end up with just Dataset for quad graphs.