Automatically download Cython

[AntoinePrv]

Cython can be automatically downloaded with setup_requires for source builds.

Right now, I have some weird errors on pip install . but I think the changes are valid and the bug is with setuptools.

FileExistsError: [Errno 17] File exists: '.../PySCIPOpt/.eggs/Cython-0.29.23-py3.9-macosx-10.9-x86_64.egg'

To follow up in the future...