Mainly just declaring official support for 3.13. Other changes are split into separate commits:

• When adding 3.12+ to CI, unittest now fails if no test cases were run, so I added a placeholder test and switched to pytest (which was already being installed)
• Migrated to pyproject.toml (the modern replacement for setup.py favoring static metadata). We can't get rid of setup.py completely due to the dynamic ptpythonX.Y entrypoints so that part remains. Also removed setup.cfg which was stale since flake8 is no longer used
• Added a dev extra for convenience when developing locally
• Fixed new UP031 violations, which is a new stabilized rule in ruff 0.8 and now causes CI to fail

Fixes #597