style: Fix linting errors in serializer tests

[aucampia]

Summary of changes

This makes it possible to remove some ignores from pyproject.toml and makes the code more PEP8 compliant.

Checklist

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

[coveralls]

coverage: 90.94%. remained the same when pulling 0729758 on aucampia:aucampia/20230830T2152-fix-linting-test_serializers into e42b752 on RDFLib:main.