MAINT: Remove Cython-generated files
MAINT: rename pyx.in -> pyx.src so that cythonize doesn't process it
BLD: generate sources using Cython at setup.py time
MAINT: include cythonize.dat into source packages

This ensures that Cython files are not regenerated if this is not
necessary. This allows also building without having Cython installed.
MAINT: tools/cythonize: fail late without Cython, use text file instead of pickle as db
MAINT: tools/cythonize: remove support for git
MAINT: include cythonize in sdist
MAINT: ship also _vq_rewrite.c

This file is unused, but include it still in sdist to allow build
without Cython installed.
MAINT: cythonize: normalize db paths
MAINT: tools/cythonize: deal with c++ Cython files
GEN: add all autogenerated .pyx files
MAINT: tools/cythonize: sort hash DB, to make sdist reproducible
BUG: make cythonization build to pass on python 3
BLD: don't require Cython to be installed for each Python version.