x11-wm/openbox: fix openbox-xdg-autostart for Python 3
Openbox includes a Python script (openbox-xdg-autostart)
which implements the XDG autostart spec
The switch of the default Python to 3.6 in ports r498529 broke the script.
The added patch makes the script work again with both Python 2 and 3.
Luckily the only problem was "print" being changed from a keyword
to a regular function.
Submitted by: Sebastian Schwarz