Cannot specify cython under setup_requires
Distribute appears to try to support Cython-based extension modules natively. Specifically, in setuptools.extension and setuptools.command.build_ext, there are specific references to support building Cython modules.
However, this support does not work when Cython is made available via setup_requires (because the tests for the existence of Cython happen before setup_requires-specified packages are loaded).
It would be nice if the package enabled installation of Cython-based packages with a simple setup_requires directive, rather than requiring it to have been installed on the build host before the target package was installed or built.