test: convert test/test_parsers/test_nquads.py to pytest

[aucampia]

Summary of changes

simple as possible conversion of test/test_parsers/test_nquads.py
to pytest.

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 remained the same at 90.486% when pulling 88a6d34 on aucampia:iwana-20220729T1408-pytest_nquads_parser into 3e2987b on RDFLib:master.

[ghost]

👍