Add cython to setup requires only if it is available
Add cython to setup requires only if it is available

Only use it to ensure that Cython is new enough if we have it available.

Signed-off-by: Sebastian Ramacher <3b98333f54c4065fc5dde7d25d9ef0c46b63a01e@ramacher.at>

sebastinas/python-libdiscid,sebastinas/python-libdiscid