Use old Cython build_ext until includes are fixed.
This change in Cython 0.25

The distutils extension Cython.Distutils.build_ext has now been updated to use cythonize which properly handles dependencies. The old extension can still be found in Cython.Distutils.old_build_ext and is now deprecated.

seems to have broken the include of auto-generated algos_common_helper.pxi. As a temporary workaround, the old build_ext can be used.
