BUG: sparse/_csparsetools: cast integer indices to same dtype
MAINT: fix cythonize treatment of .pyx.in files; use Cython-bundled Tempita
BUG: sparse: rewrite _csparsetools using Tempita rather than fused types

As of Cython 0.20.1, the fused types dispatch mechanism does not seem
solid enough, so we fall back here to a simpler solution.
MAINT: fix cythonize/tempita for Py3
Merge pull request #3476 from pv/csparsetools-fixes

MAINT: use custom templating instead of fused types in _csparsetools