[MRG+2] BUILD Rewrite setup.py files to handle cython dependencies (#7719)

* Rewriting of cythonization in setup.py

By using Cython.Build.cythonize and switching between .c and .pyx files
as appropriate cython dependencies are correctly taken into account.

* Use cythonize once on the root config rather than in each subpackage

* Fix for Windows

* Remove caching from Travis

Cython dependencies are taken care of by Cython.Build.cythonize and
based on file timestamps, so .C and .so files will always be rebuild
from scratch on each build in Travis.

* Specify .pyx in setup.files for cython generated extensions

More natural this way. Tweak the extensions to generate from .c and .cpp
files for a release.

* COSMIT Remove commented out code

* Check cython version is greater than 0.23

* COSMIT better names for functions

* flake8 fix (imported module not at top of file)

* Install cython 0.23 for Python 2.6

now that cython >= 0.23 requirement is enforced in setup.py

* Use module constant for minimum required cython version

* Fix Travis install.sh

No easy way to put comments inside multi-line command