Fix py35 test failure

dict.keys() returns object dict_keys on Python 3, can't work well
with kombu recently, convert it to list before futhur processing.

Closes-Bug: #1687855
Change-Id: I9dcfadd8f16eccce06f4cde3b2a6669f28a3f8bc

taskflow/engines/worker_based/server.py

@@ -147,7 +147,7 @@ class Server(object):
                      ku.DelayedPretty(message), exc_info=True)
         else:
             response = pr.Notify(topic=self._topic,
-                                 tasks=self._endpoints.keys())
+                                 tasks=list(self._endpoints.keys()))
             try:
                 self._proxy.publish(response, routing_key=reply_to)
             except Exception: