set some topics on IOPub messages

these are informally defined and have no effect,
but we should be a bit better with them.

IPython/kernel/zmq/displayhook.py

@@ -10,7 +10,7 b' from session import extract_header, Session'
 class ZMQDisplayHook(object):
     """A simple displayhook that publishes the object's repr over a ZeroMQ
     socket."""
-    topic=None
+    topic=b'pyout'
 
     def __init__(self, session, pub_socket):
         self.session = session

IPython/kernel/zmq/iostream.py

@@ -44,6 +44,7 b' class OutStream(object):'
         self.session = session
         self.pub_socket = pub_socket
         self.name = name
+        self.topic = b'stream.' + py3compat.cast_bytes(name)
         self.parent_header = {}
         self._new_buffer()
         self._buffer_lock = threading.Lock()

IPython/kernel/zmq/zmqshell.py

@@ -61,7 +61,7 b' class ZMQDisplayPublisher(DisplayPublisher):'
     session = Instance(Session)
     pub_socket = Instance(SocketABC)
     parent_header = Dict({})
-    topic = CBytes(b'displaypub')
+    topic = CBytes(b'display_data')
 
     def set_parent(self, parent):
         """Set the parent for outbound messages."""