Timeout callback of request.

        Construct a timeout HTTPResponse when a timeout occurs.

        :arg object key: A simple object to mark the request.
        :info string key: More detailed timeout information.

def _on_timeout(self, key: object, info: str = None) -> None:
        """Timeout callback of request.

        Construct a timeout HTTPResponse when a timeout occurs.

        :arg object key: A simple object to mark the request.
        :info string key: More detailed timeout information.
        """
        request, callback, timeout_handle = self.waiting[key]
        self.queue.remove((key, request, callback))

        error_message = "Timeout {0}".format(info) if info else "Timeout"
        timeout_response = HTTPResponse(
            request,
            599,
            error=HTTPTimeoutError(error_message),
            request_time=self.io_loop.time() - request.start_time,
        )
        self.io_loop.add_callback(callback, timeout_response)
        del self.waiting[key]