Remove specified host/port or clear all dns local cache.

def clear_dns_cache(self,
                        host: Optional[str]=None,
                        port: Optional[int]=None) -> None:
        """Remove specified host/port or clear all dns local cache."""
        if host is not None and port is not None:
            self._cached_hosts.remove((host, port))
        elif host is not None or port is not None:
            raise ValueError("either both host and port "
                             "or none of them are allowed")
        else:
            self._cached_hosts.clear()