class_docstr: Extend Dask's Client with handlers to handle PubSub machinery
code: def cleanup(self):
      for name, s in self.subscribers.items():
          if not s:
              msg = {"op": "pubsub-remove-subscriber", "name": name}
              self.client.scheduler_comm.send(msg)
  
imports: from __future__ import annotations
  import asyncio
  import logging
  import threading
  import weakref
  from collections import defaultdict, deque
  from dask.utils import parse_timedelta
  from distributed.core import CommClosedError
  from distributed.metrics import time
  from distributed.protocol.serialize import to_serialize
  from distributed.utils import TimeoutError, sync, wait_for
label_desc: Use image node_id 3 for calling the PubSubClientExtension obj's underlying member method code with example usage: obj.cleanup() without return types