Fix testing for warnings with pytest 8 (#2748)

[nphilipp]

New versions of pytest don’t catch log.warning() as a UserWarning. Use the caplog fixture instead.

Related: #2727 Fixes: #2748

Summary of changes

Use the caplog fixture instead of pytest.warns() to check for warnings emitted with log.warning(), as outlined in https://github.com/RDFLib/rdflib/pull/2727#issuecomment-1994503300

Checklist

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

[coveralls]

coverage: 91.039% (+0.009%) from 91.03% when pulling 03bb7c11d61db79ec6b5f1245733da26c9867355 on nphilipp:main--pytest8-compat into 0ecc40009ae397c2798c0c08a2d751a1a9d2f8a7 on RDFLib:main.

[nicholascar]

Thanks @nphilipp!