Convert Define into an enum

[multimeric]

Summary of changes

Convert Defined into an enum. The justification of this is PEP 484, which suggests that singletons should be represented as enums with a single element. The reason why this matters is because it lets you say if x is not UNDEF and the type checker will correctly narrow the type to be no longer Defined, because all enum variants have been excluded.

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]

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