`auth` tuple. If the value is `None`, it will be
                              omitted from the request, however if the value is
                              an empty string, an empty string will be sent.
        :param cert: Client certificate to send for OAuth 2.0 Mutual-TLS Client
                     Authentication (draft-ietf-oauth-mtls). Can either be the
                     path of a file containing the private key and certificate or
                     a tuple of two filenames for certificate and key.
        :param kwargs: Extra parameters to include in the token request.
        :return: A token dict
        ©