But this down for my future reference:
If you want to test a custom Cython with Sage, do the following.
1. Download and install Sage from source or a binary.
http://sagemath.org/download.html
If from source, download the tarball here, extract it, and type make,
then wait 2 hours:
http://sagemath.org/dist/src/
2. After Sage is done building, go to your personal Cython, and type
sage -python setup.py install
This will install *your* Cython into the Python that ships with Sage.
3. From the Sage root directory type
./sage -ba
This will rebuild all the Cython code in Sage (well over 50,000 lines
of hand-written Cython code), which should take about 15-20 minutes.
4. Assuming 3 finishes, from the Sage root directory type
make check
This will run the entire Sage test suite (over 50,000 input examples),
and report the result.
-- William [Stein]