Cope with Namespace annotations in Python 3.14

[nphilipp]

I submitted this already in #3084 which got merged, but the change is missing from the main branch. So here we go again:

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The annotations member can be incomplete, use the get_annotations() helper from annotationlib (Python >= 3.14) or inspect (Python >= 3.10) if available.

Related: #3083

Summary of changes

This fixes accessing Namespace annotations on Python 3.14, which makes import rdflib fail on this Python version. This should be backwards-compatible.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes. ⇒ Other tests (sparql) fail on Python 3.14, see Failures with Python 3.14 #3083
• If the change adds new features or changes the RDFLib public API: n/a
• If the change has a potential impact on users of this project: n/a (covered by existing tests)
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.