only run cython in setup.py if we need to

[tacaswell]

As of #80 cython is run unconditionally in setup.py which means you need cython install to create the sdist. Can probably do something like https://github.com/h5py/h5py/blob/4589052eaa6aa8fec612bb2b73c11006e7ead20f/setup.py#L44-L45