testing/py3-gpep517: upgrade to 9

https://github.com/mgorny/gpep517/compare/v8...v9

• Support --optimize in install-wheel
• Remove redundant "/usr" prefix fallback
• test: Verify that .pyc files have correct source .py
• Commonize install path options
• Integrate build-backend fallback in build-wheel
• Group arguments and improve help text
• Actually make flit_core test dep optional
• Introduce an install-from-source command
• Extend integration tests to install-from-source command
• Add progress reporting
• Make more profound use of pathlib.Path