docs: def _ensure_worker(self):
          """Ensure there are enough workers available"""
          while len(self._workers) < self._min_workers or len(self._workers) < self._queue.qsize() < self._max_workers:
              worker = threading.Thread(
                  target=self._execute_futures,
                  name=self.identifier + '_%d' % time.time(),
              )
              worker.daemon = True
              self._workers.add(worker)
              worker.start()