[Cython] Sage build broken on Jenkins

[Stefan Behnel]

Hi,

the Sage build has been broken for a while now. Might be due to changes in
Sage, or due to one of the (minor?) cythonize() changes in master.

https://sage.math.washington.edu:8091/hudson/job/sage-build/

Build error message is:

"""
IOError: could not find dependency sage/calculus/numpy.pxd included in
sage/calculus/riemann.pyx.
"""

https://sage.math.washington.edu:8091/hudson/job/sage-build/1819/console

Stefan