package: add python3 support in the package infrastructure

This commit improves the Python package infrastructure to allow Python
packages to be built with Python 3. The changes are fairly simple:

 * Use either PYTHON_PATH or PYTHON3_PATH as the PYTHONPATH depending
   on which Python is used.

 * Depend on host-python or host-python3 and python or python3
   depending on which Python is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>