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 <firstname.lastname@example.org>