Ensure GTK+ version 3 is imported in cairo_draw.py

src/graph_tool/draw/cairo_draw.py

@@ -1453,6 +1453,8 @@ def get_hierarchy_control_points(g, t, tpos, beta=0.8, cts=None, is_tree=True,
 #
 
 try:
+    import gi
+    gi.require_version('Gtk', '3.0')
     from gi.repository import Gtk, Gdk, GdkPixbuf
     from gi.repository import GObject as gobject
     from .gtk_draw import *