Fix gtk-demo for Python 3
Fix gtk-demo for Python 3

It was crashing with

   package = __import__(packagename, globals(), locals(), [modulename], -1)
  ValueError: level must be >= 0

Using level zero works with both Python 2 and 3.

GNOME/pygobject,GNOME/pygobject,sfeltman/pygobject,sfeltman/pygobject,davibe/pygobject,pexip/pygobject,pexip/pygobject,davibe/pygobject,pexip/pygobject,davibe/pygobject,davibe/pygobject,sfeltman/pygobject,GNOME/pygobject