Stop including generated Cython code in source distributions.

Ensures that code will always be generated when building for a given
target environment, this is because Cython generates code which accesses
CPython internal symbols and we cannot guarantee that the C code we
generated will be valid for the target CPython interpretor.


  o Don't include "*.c"

  o Dont cythonize for "sdist" target

  o tox.ini: Include cython explicitly for the linting target
3 files changed