Port zmq driver to Python 3

With eventlet 0.16, it becomes possible to run Oslo Messaging tests on
Python 3 with eventlet.

This change ports the zmq driver to Python 3:

* encode the topic explicitly to UTF-8
* use a list comprehension instead of map() to also get a list
  on Python 3 (not a generator)

The following eventlet change is needed to run tests:
https://github.com/eventlet/eventlet/pull/187

Related eventlet issue:
https://github.com/eventlet/eventlet/issues/185

I will propose a different change to enable tests with eventlet enabled
when a release of eventlet including this fix will be available.

Change-Id: Ic8fec515cfa757e08ffb9604e3bfb2e87d08f3d8

oslo_messaging/_drivers/impl_zmq.py

@@ -243,14 +243,17 @@ class ZmqClient(object):
         msg_id = msg_id or 0
 
         if not envelope:
-            self.outq.send(map(bytes,
-                           (msg_id, topic, 'cast', _serialize(data))))
+            data = _serialize(data)
+            if six.PY3:
+                data = data.encode('utf-8')
+            data = (msg_id, topic, b'cast', data)
+            self.outq.send([bytes(item) for item in data])
             return
 
         rpc_envelope = rpc_common.serialize_msg(data[1])
         zmq_msg = moves.reduce(lambda x, y: x + y, rpc_envelope.items())
-        self.outq.send(map(bytes,
-                       (msg_id, topic, 'impl_zmq_v2', data[0]) + zmq_msg))
+        data = (msg_id, topic, b'impl_zmq_v2', data[0]) + zmq_msg
+        self.outq.send([bytes(item) for item in data])
 
     def close(self):
         self.outq.close()
@@ -453,6 +456,8 @@ class ZmqProxy(ZmqBaseReactor):
 
         data = sock.recv(copy=False)
         topic = data[1].bytes
+        if six.PY3:
+            topic = topic.decode('utf-8')
 
         if topic.startswith('fanout~'):
             sock_type = zmq.PUB
@@ -658,6 +663,8 @@ def _cast(addr, context, topic, msg, timeout=None, envelope=False,
     allowed_remote_exmods = allowed_remote_exmods or []
     timeout_cast = timeout or CONF.rpc_cast_timeout
     payload = [RpcContext.marshal(context), msg]
+    if six.PY3:
+        topic = topic.encode('utf-8')
 
     with Timeout(timeout_cast, exception=rpc_common.Timeout):
         conn = None