e.auth.transport.requests.Request):
            (Optional) An instance of
            :class:`~google.auth.transport.requests.Request` used when
            refreshing credentials. If not passed,
            an instance of :class:`~google.auth.transport.requests.Request`
            is created.
        default_host (Optional[str]): A host like "pubsub.googleapis.com".
            This is used when a self-signed JWT is created from service
            account credentials.
    Nc