Fix PYTHONPATH on Windows
On Windows, PYTHONPATH is set to $CERBERO_PREFIX/Lib/site-packages by default but some libraries use a versioned PYTHONPATH for their output (pycairo or gi for example, which install in $CERBERO_PREFIX/lib/python3.7/site-packages like on a posix system).
This patch adds an extra path to PYTHONPATH on Windows platform using the same scheme as on a posix system.