Call me at checkConfig time to properly report config error
           if neither txrequests or treq is installed

def checkAvailable(from_module):
        """Call me at checkConfig time to properly report config error
           if neither txrequests or treq is installed
        """
        if txrequests is None and treq is None:
            config.error("neither txrequests nor treq is installed, but {} is requiring it\n\n{}".format(
                from_module, HTTPClientService.TREQ_PROS_AND_CONS))