py3: fix schema response for py2 server with py3 client
    
    The schema RPC response can include a bytes value for the
    'topic_topic' field (when the server is running in Python 2).
    In this case, a Python 3 client fails to initialise the API
    when a schema fetch is required, because it cannot serialise
    the bytes values in the schema object to JSON.
    
    Ensure the 'topic_topic' value is unicode.
    
    Part of: https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>