fix: Dataset.parse now returns Self

[edmondchuc]

Summary of changes

Both ConjunctiveGraph and Dataset's parse method now correctly returns Self like their superclass Graph.

The test confirms that now the return value has the same reference as the original dataset that calls parse.

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.

[edmondchuc]

The doctest is updated to the correct class type now that ConjunctiveGraph's parse method returns Self.