refactor: narrow imports

[aucampia]

Summary of changes

This change narrows import so that things are imported from the Python module where they are defined instead of importing them from a module that re-exports them, e.g. change import of Graph to import from the rdflib.graph module instead of from the rdflib module. This helps avoid problems with circular imports.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.831% (-0.0004%) from 90.832% when pulling eec6c24 on aucampia:iwana-20230409T1848-narrow_imports into 4940798 on RDFLib:main.

[aucampia]

Merging without review, as this does not change runtime behaviour.