enforce Cython
    
    This falls under pre-generated code, while autotools pregenerated
    code is used only in build-time, Cython pregenerated code is
    what you get for runtime and there might be vulnerabilities
    or bad performance due to that (depends on which Cython version
    upstream generated C code). Also upstream had to release 4.1.1
    with rebuilt C code with latest Cython due to Py3.7.
    
    Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>