python: Fix xmlrpclib imports.

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>