celery: use just one instance of UNSET() object

rhodecode/lib/celerylib/loader.py

@@ -246,7 +246,10 b' class UNSET(object):'
     pass
 
 
-def set_celery_conf(app=UNSET(), root=UNSET(), request=UNSET(), registry=UNSET(), closer=UNSET()):
+_unset = UNSET()
+
+
+def set_celery_conf(app=_unset, root=_unset, request=_unset, registry=_unset, closer=_unset):
 
     if request is not UNSET:
         celery_app.conf.update({'PYRAMID_REQUEST': request})