Hi,
I also strongly support this initiative for Sage to join NumFocus at this point in time.
Regarding Cython, I made up the name in maybe 2008, and started it as a project by combining "Pyrex" that Greg Ewing worked on periodically,
my own fork of Pyrex that had features I added that were needed for Sage, and Stefan Behnel's fork that was needed for lxml.
Robert Bradshaw (my phd student at the time) and Craig Citro (my postdoc) then started working on it, and they added a huge amount
of functionality, e.g., they fully implemented closures, which was nontrivial; also, I remember they added some type
inference optimizations, and generally greatly improved the unit testing. I've cc'd Robert and Craig in case they want to add anything.
Sage has always been I think by far the biggest Cython user, and testing of Cython by Sage helps ensure better stability and quality.
-- William