Fix issue 1808

[edmondchuc]

Summary of changes

Fixes #1808

Essentially we are reverting back to returning a URIRef in the URIRef.skolemize() method and changing how Graph.de_skolemize() method converts subjects and objects of a statement to a BNode.

I think this change is much safer than changing what is returned by URIRef.skolemize() and potentially affecting other parts of the library. I've had a search through the codebase and nothing else touches the Graph.de_skolemize().

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• For changes that have a potential impact on users of this project:
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
  • Considered adding an example in ./examples for new features.
  • Considered updating our changelog (CHANGELOG.md).
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[aucampia]

@edmondchuc - pre commit errors can be fixed by making a comment "pre-commit.ci autofix"

[nicholascar]

pre-commit.ci autofix

[edmondchuc]

Thanks for the tip @aucampia and thanks @nicholascar for commenting.

I'm hoping this PR fixes both #1808 and #1404 without introducing further side effects.

[ghost]

Nice work.

[aucampia]

Will review in about an hour, just fixing some other issues.

[edmondchuc]

pre-commit.ci autofix

[aucampia]

@edmondchuc I have added some more tests now - and I'm fairly comfortable with the change after adding them - hope they are okay with you.

[aucampia]

Fix looks good, thanks @edmondchuc

[aucampia]

Planning to merge on or before 2022-04-17 if there is no further feedback.

[edmondchuc]

@edmondchuc I have added some more tests now - and I'm fairly comfortable with the change after adding them - hope they are okay with you.

Thanks for adding the additional tests @aucampia.

[aucampia]

Won't merge before #1838