Fix logic for compiling from Cython files

At the moment, ``release=False`` in the commits where the version is set to a release version (because it does not end with ``.dev``. However, we never include the Cython files in the git repo, but ``setup.py`` will look for the ``.c`` files if ``release==True``, so one cannot succesfully build the package from the git repository when the version string does not end with ``.dev``. This causes issues with readthedocs for example.

One solution is to have a custom sdist like this:

https://github.com/hyperion-rt/hyperion/blob/master/setup.py#L26

that will replace ``release=False`` in the git repo by ``release=True`` only in the tar file. I can open a PR if other people think this is a good idea.