python3-cython: Add new recipe

Use a common .inc file for the python-cython and python3-cython recipes.
Add suffix '3' to scripts in ${bindir} to avoid conflict between the two
packages.

Signed-off-by: Dominic Sacré <dominic.sacre@gmx.de>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>