chore: remove cython from requirements

[Midnighter]

I didn't see instructions regarding whether the PR should be merged into master or develop.

Fix #898

Remove Cython from the package requirements since it's only needed at installation time and already listed in pyproject.toml build requirements.

I didn't make any other changes here but I did notice that the whole package installation/building configuration uses outdated mechanisms. You may consider making further changes:

• Setuptools now fully supports pyproject.toml. This would let you get rid of setup.cfg and pytest.ini as well.
• Supporting python setup.py test is not typically done anymore.
• You can pre-compile your .pyx modules into .c ones and make those part of your source distribution instead. In that case, cython wouldn't even be needed at installation time but only when making your sdist.

[wasade]

Thanks, @Midnighter! And thank you for the additional notes on items that we can be improved! I usually only get brief windows of time for maintenance once or twice a year, and I've preferentially focused on bugs / functionality / performance. That said, I would like to spend a cycle at some point on project-level maintenance.

[Midnighter]

That's... not very often 😅

[wasade]

Ya :) but the items you raise are important and I would like to make sure we keep forward progress (even if slow) rather than stalling out.