Remove compiled Cython files from sdist
NumPy is considering this in https://github.com/numpy/numpy/pull/14453. I suspect other projects will follow suite.

The tldr is that as long as we ship the cythonized code, our older sdists have no hope of working with newer Pythons. The have to be compiled with a modern enough Cython.

I think this makes even more sense for us. We already require that NumPy is present as a build dependency. Requiring Cython on top of that is not a big deal.

Do we want to do this for 0.25.2?