Our type hint for "let" is incorrect because they're missing generics:

$ git grep '\[Mapping\]'
pymongo/asynchronous/collection.py:707:        let: Optional[Mapping] = None,
pymongo/asynchronous/collection.py:3205:        let: Optional[Mapping] = None,
pymongo/asynchronous/mongo_client.py:2405:        let: Optional[Mapping] = None,
pymongo/synchronous/collection.py:706:        let: Optional[Mapping] = None,
pymongo/synchronous/collection.py:3198:        let: Optional[Mapping] = None,
pymongo/synchronous/mongo_client.py:2393:        let: Optional[Mapping] = None,

We should update them to something like: "let: Optional[Mapping[str, Any]] = None,"

A user commented on how pyright raises errors on partially unknown types (for example Mapping and List instead of Mapping[str, Any] and List[int])

It would be nice to eventually turn on the appropriate flags in mypy (disallow_any_generics) and pyright (reportMissingTypeArgument) that enforces typed containers.

As of the creation of this ticket, turning on the flag in pyright seemed to produce ~250 errors (and i'd guess about half of these errors are duplicated sync errors)