code: def checkAvailable(from_module):
          
          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))
docs: Call me at checkConfig time to properly report config error
             if neither txrequests or treq is installed