code: def _get_retrier(self, receiver: Address) -> _RetryQueue:
          
          if receiver not in self._address_to_retrier:
              retrier = _RetryQueue(transport=self, receiver=receiver)
              self._address_to_retrier[receiver] = retrier
              # Always start the _RetryQueue, otherwise `stop` will block forever
              # waiting for the corresponding gevent.Greenlet to complete. This
              # has no negative side-effects if the transport has stopped because
              # the retrier itself checks the transport running state.
              retrier.start()
          return self._address_to_retrier[receiver]
docs: Construct and return a _RetryQueue for receiver