add support for python 3.13

[branchv]

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

[ches]

@jonathanslenders Might you have some time for review? Thanks @branchvincent!

[jonathanslenders]

Sorry, I missed this PR, and did about the same changes here: #603

[jonathanslenders]

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

Actually, I think this is a bug even when wheels are created and ptpython is installed through a wheel. The entry_points.txt inside the dist-info within the wheel contains entry points that refer to the python version used to build the wheel. Not the Python version for which the wheel is installed.

So, I think we should get rid of that anyway.

@branchvincent : If you have time and don't mind, could you rebase this on the "main" branch, removing setup.py and setup.cfg in favor of pyproject.toml?

[jonathanslenders]

@branchvincent : I cherry-picked the remaining changes and merged it as part of #604
I think we can close this now.