prefs2prefs.py not compatible with python3

[pdv]

The current master fails to start-up (on macOS) if the default Python is 
Python3.x, because prefs2prefs.py is not compatible with Python3.

In python23() in os.cpp both Python 2 and Python 3 are accepted.

As a temporarily workaround I've built with PYTHON="/usr/bin/python" 
which is the default apple-python, but that doesn't seem to help.

P. De Visschere