Upgrade Cython version for Python 3.7 compatibility

[sputnick1124]

Basically a duplicate of cython/cython#1955 which has propagated to here. Using cython==0.28.5 to generate _cython_speedups.cpp (sputnick1124@eee418d) builds fine on 3.7.

[sputnick1124]

Sister issue for chaco: enthought/chaco#426

[silverjam]

This should be fixed by #336

[sputnick1124]

Thanks!