Do not include Cython generated files in sdist

The recommendation in the Cython documentation has changed:

Following recent improvements in the distribution toolchain, it is not recommended to include generated files in source distributions. Instead, require Cython at build-time to generate the C/C++ files, as defined in PEP 518 and PEP 621.

See https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html#distributing-cython-modules

The presence of the C file causes conflicts when building .deb packages because the C file that is created while building the .deb package can be different depending on the Cython version being used.