listener: Move glib import into function

Related to #1068

mopidy/listener.py

@@ -2,14 +2,18 @@ from __future__ import absolute_import, unicode_literals
 
 import logging
 
-import gobject
-
 import pykka
 
 logger = logging.getLogger(__name__)
 
 
 def send_async(cls, event, **kwargs):
+    # This file is imported by mopidy.backends, which again is imported by all
+    # backend extensions. By importing modules that are not easily installable
+    # close to their use, we make some extensions able to run their tests in a
+    # virtualenv with global site-packages disabled.
+    import gobject
+
     gobject.idle_add(lambda: send(cls, event, **kwargs))