In 50python-mode-gentoo.el, we already default to python-mode for *.py files. Cython[1] files are python-mode compatible and use the 'pyx' extension, so it would be convenient to auto-mode them as well. [1] http://docs.cython.org/src/quickstart/build.html
Added with version bump to Python-mode 6.1.0. Thanks for the hint.