se processing.

        'mechanism' specifies which authentication mechanism is to
        be used - it must appear in <instance>.capabilities in the
        form AUTH=<mechanism>.

        'authobject' must be a callable object:

                data = authobject(response)

        It will be called to process server continuation responses; the
        response argument it is passed will be a bytes.  It should return bytes
        data that will be base64 encoded and sent to the server.  It should
        return None if the client abort response '*' should be sent instead.
        r