python3 is not supporting gtk module
Solution 1:
try:
from gi.repository import Gtk
and replace gtk by Gtk in your code
or
from gi.repository import Gtk as gtk
see also : Python GTK+ 3 Tutorial : Getting Started
Solution 2:
Use the GObject introspection based Python3 bindings for Gtk and friends:
from gi.repository import Gtk, GObject
That needs the package python3-gi which is installed by default.
Some names have changed since PyGTK. The Python GObject Introspection API Reference should help you to find the new names (and other changes).