Xpra: Ticket #1891: Python3 build installs python2 scripts

Pyyhon3 build installs the following python2 scripts:

usr/lib/cups/backend/xpraforwarder
usr/lib/xpra/auth_dialog
usr/lib/xpra/xdg-open

That is incorrect. This change must come from your build or packaging system:

$ sudo python3 ./setup.py install --prefix=/usr
(..)
$ sudo head -n 1 /usr/lib/cups/backend/xpraforwarder
#!/usr/bin/env python

Notes:

r19776 fixes the python3 compatibility for xpraforwarder
xdg-open and xpraforwarder will just end up exec-ing xpra, so they do not rely on any xpra modules directly - they should be able to run with both python2 or python3
xdg-open and xpraforwarder could be re-written in plain shell relatively easily
auth_dialog imports some xpra modules for its dialog (mostly the utility module that makes it GTK2 / GTK3 and python2 / python3 version agnostic), so it requires that the corresponding xpra modules are installed for the python version used to run it