python: Fix xmlrpclib imports.

Fix imports of xmlrpclib to be compatible with Python 3.  Python 2 had
xmlrpclib (client) and SimpleXMLRPCServer (server).  In Python 3, these
have been renamed to xmlrpc.client and xmlrpc.server.

The solution implemented here is to use the six library.  It may seem
excessive for this particular issue, but the six library provides
helpers for Python 2 and 3 compatibility for many different issues.
This is just the first of many uses of the six library.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

python/ovstest/rpcserver.py

@@ -20,8 +20,8 @@ from __future__ import print_function
 
 import exceptions
 import sys
-import xmlrpclib
 
+import six.moves.xmlrpc_client
 from twisted.internet import reactor
 from twisted.internet.error import CannotListenError
 from twisted.web import xmlrpc
@@ -108,7 +108,8 @@ class TestArena(xmlrpc.XMLRPC):
         Returns the ovs-test server IP address that the other ovs-test server
         with the given ip will see.
         """
-        server1 = xmlrpclib.Server("http://%s:%u/" % (his_ip, his_port))
+        server1 = six.moves.xmlrpc_client.Server("http://%s:%u/" %
+                                                 (his_ip, his_port))
         return server1.get_my_address()
 
     def xmlrpc_create_udp_listener(self, port):