skip prefix generation for predicates corresponding to base namespace

[bulricht]

Summary of changes

Small fix to prevent the turtle serializer from generating prefixes when predicates contain the base uri.
cfr. issue #3237

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 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]

Thanks @bulricht, could you add a test case that demonstrates the fix?

[coveralls]

coverage: 90.298% (-0.004%) from 90.302%
when pulling 8cb7ba7 on bulricht:main
into 4a819fa on RDFLib:main.

[edmondchuc]

Closing this PR in favour of #3244 to target the 7.x branch instead of main.