Patch for Gtk w/o introspection support

python setup.py install --user
fails with
AttributeError: 'gi.repository.Gtk' object has no attribute 'get_major_version'

for old versions of Gtk without support for introspection
(specifically, on Ubuntu 12 with python-gtk2 2.24.0-3)

This commit silently drops the associated exceptions for the two Gtk
version checks in setupext.py