Figure out correct setup.py and pyproject.toml

[segevfiner]

So I tried to use setuptools' built-in support for Cython. Oh foolish me...

By including Cython in pyproject.toml/build-system.requires, distributing the C source file is kinda pointless as it will always be regenerated by the installed Cython as far as I can understand. The documentation is really confusing on the subject.

See https://groups.google.com/g/cython-users/c/C52mc9lm5xo

[segevfiner]

Arghhh, so I still include cython in pyproject.toml even though it won't be needed for an sdist because otherwise pip won't install Cython before running the setup.py in development...