Close topology events executor faster after the topology is garbage collected

Topology's events executor is not closed as soon as it could be when the topology is garbage collected: https://github.com/mongodb/mongo-python-driver/blob/3.8.0/pymongo/topology.py#L116-L131

The weakref.ref is defined like this:

            # We strongly reference the executor and it weakly references

            # the queue via this closure. When the topology is freed, stop

            # the executor soon.

            weak = weakref.ref(self._events)

It should pass the executor's close method:

            weak = weakref.ref(self._events, executor.close)

Without executor.close the Topology events executor will still be closed but it will wait until the executor actually attempts to run (1 second instead of 0.5 seconds).