Fix the uses of stanza.reply()

This is relying on the stanzas being copied for each handler. We no
longer do that for performance reasons, so instead of editing the copy
in-place, stanza.reply() now returns a new stanza.

slixmpp/plugins/xep_0027/gpg.py

@@ -67,8 +67,7 @@ class XEP_0027(BasePlugin):
         register_stanza_plugin(Message, Encrypted)
 
         self.xmpp.add_event_handler('unverified_signed_presence',
-                self._handle_unverified_signed_presence,
-                threaded=True)
+                self._handle_unverified_signed_presence)
 
         self.xmpp.register_handler(
                 Callback('Signed Presence',