Move XEP-0172 to new system.

sleekxmpp/plugins/xep_0172/__init__.py

@@ -6,6 +6,11 @@
     See the file LICENSE for copying permission.
 """
 
+from sleekxmpp.plugins.base import register_plugin
+
 from sleekxmpp.plugins.xep_0172 import stanza
 from sleekxmpp.plugins.xep_0172.stanza import UserNick
-from sleekxmpp.plugins.xep_0172.user_nick import xep_0172
+from sleekxmpp.plugins.xep_0172.user_nick import XEP_0172
+
+
+register_plugin(XEP_0172)