[Cython] Enforce usage of cython for sdist archive creation and when on ...

We're only requiring Cython now during setup, if invoked from a git working copy (we do not want to ship generated source code) or we're creating a source archive with sdist (which is uploaded to pypi etc.), since we do not want to enforce those users to have Cython installed.

tests run: 
* on git, no cython -> installs fine
* create sdist, ensure install works without cython