CI: add validate_unwanted_patterns to known_third_parties
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

By using `seed-isort-config`, the `known_third_party` setting is automatically populated and alphabetically sorted. This is a cleaner solution which doesn't require manually adding third parties to `setup.cfg`.

Furthermore, it catches an existing inconsistency: currently, `validate_docstrings` is set as a known third party, while `validate_unwanted_patterns` isn't. Neither should be, IMO.