Support for pymongo > 4

Looking at the setup.py it seems that pymongo>4 is supported (and the only version supported)

https://github.com/hyperopt/hyperopt/blob/e9122b53041bf4bddff402ab6df251d379cf3420/setup.py#L60

However this is no the case, as there are several functions from < 4 that this library uses and they have been removed. e.g., count https://github.com/hyperopt/hyperopt/blob/e9122b53041bf4bddff402ab6df251d379cf3420/hyperopt/mongoexp.py#L859

https://pymongo.readthedocs.io/en/stable/migrate-to-pymongo4.html#collection-count-and-cursor-count-is-removed

Is there any plans to update hyperopt to work with pymongo>=4? Otherwise the setup.py should be updated I believe.