bug#19908: python.el: minimal cython indentation support

[Nathan Weston]

I've encountered similar problems writing a mode for Enaml.

What I really want is to override python-rx for enaml-mode, while 
leaving the original definition for python-mode. But that doesn't seem 
to be possible.

For Enaml, I was able to get indentation working by advising 
python-indent-context, but it's not particularly elegant. And other 
function which use (python-rx 'block-start) don't work correctly in 
enaml-mode.