Not working with i3

I'm able to start `snaptile.py`, but then I get the following error after any keybinding (for example `Ctrl + Alt + e`):

```

Snaptile running. Press CTRL+C to quit.
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/gi/overrides/GLib.py", line 682, in
func_fdtransform = lambda _, cond, *data: callback(channel, cond, *data)
File "./snaptile.py", line 86, in checkevt
handleevt(event.detail, event.detail)
File "./snaptile.py", line 105, in handleevt
posmap[endkey]
File "/home/snaptile/window.py", line 4, in position
window, screen = active_window()
File "/home/snaptile/window.py", line 31, in active_window
if no_window(screen, window):
File "/home/snaptile/window.py", line 45, in no_window
not screen.supports_net_wm_hint(
AttributeError: 'gtk.gdk.X11Screen' object has no attribute 'supports_net_wm_hint'
```

Start by running snaptile.py under i3 and triggering a keybinding such as Ctrl + Alt + e. Trace the call from checkevt and handleevt in snaptile.py into position, active_window, and no_window in window.py, where the missing supports_net_wm_hint attribute is reported. Done means keybindings work without this traceback.

Written by the indexing model from the issue text.