Comment 4 for bug 1576193

Not sure how to send a patch, I'm used to git
This is all new to me :)

What I did was to add next lines in "/usr/lib/python3/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py"

from gi import require_version
require_version('Gtk', '3.0')
require_version('Gdk', '3.0')

before the next line

from gi.repository import GObject, Gdk, Gtk, Gio, GLib