Switch to cython pure python mode

Switch to cython pure python mode

Replaces the pyx files with py files that can be both compiled
by cython or imported as is by python.
This avoids the need of duplicating the code to have a python
only fallback.
The cython files are also reorganized to be in the module they use
instead of all being in the cyextension package, that has been
removed.

The performance is pretty much equal between main and this change.
A detailed comparison is at this link
https://docs.google.com/spreadsheets/d/1jkmGpnCyEcPyy6aRK9alElGjxlNHu44Wxjr4VrD99so/edit?usp=sharing

Change-Id: Iaed232ea5dfb41534cc9f58f6ea2f912a93263af