Use Extension from distutils in setup.py

It turns out we don't actually need Extension from Cython.Distutils.
The extension module builds fine with Extension from standard
distutils. In fact using Cython.Distutils.Extension requires a very
recent version of distribute which wasn't available by default on
recent Ubuntu systems and led to an awkward and hard to debug error.

[Buildbot pass](http://buildbot-ocean.ese.ic.ac.uk:8080/builders/pyop2-testing/builds/127)