core/desktop: don't crash if gi exists but is empty (#314)

Many packages install something into gi/overrides. If the gobject
package itself is not installed, the import of gi succeeds but
require_version is not contained within, resulting in an AttributeError.

i3pystatus/core/desktop.py

@@ -57,7 +57,7 @@ try:
 
     gi.require_version('Notify', '0.7')
     from gi.repository import Notify
-except (ImportError, ValueError):
+except (ImportError, ValueError, AttributeError):
     pass
 else:
     if not Notify.init("i3pystatus"):