Context

External contribution in https://github.com/mongodb/motor/pull/272.

"This PR adds generics to type stubs to match pymongo. For example, MongoClient is generic on its _DocumentType. This PR makes AgnosticClient, AsyncIOMotorClient, and MotorClient also generic on the same _DocumentType (as well as all the other classes where the corresponding pymongo class is generic, including Database, Collection, ChangeStream, and the various Cursors)."

Definition of done

Generics are added and tested.

Pitfalls

The semantics of generics may change slightly with PYTHON-3992, so we may have to update tests and add a note about the change after we adopt it in pymongo.