I think it would be better to discourage projects from including the output of cython in their sdists.  They should either have cython as a build-time requirement or provide built wheels (which are specific a platform and CPython version).  The middle ground of not expecting the user to have cython while expecting them to have a working c-complier is a very narrow case and I think asking those users to install cython is worth the forward compatibility for Python versions you get by requiring people installing from source to re-cythonize.