TST Add defusedxml to tox deps

[hoodmane]

Otherwise tests fail with: ImportError: Error importing plugin "sphinx.testing.fixtures": No module named 'defusedxml'.

See: https://github.com/sphinx-doc/sphinx/issues/12339

Tasks

• [x] Added unit tests
• [x] Added documentation for new features (where applicable)
• [x] Added release notes (using reno)
• [x] Ran test suite and style checks and built documentation (tox)

[stephenfin]

Thanks :pray: