It is unclear why any given MOTOR release supports a range of PyMongo versions as opposed to a single PyMongo version (the latest). The PHP driver, for instance, always bumps its dependency on libmongoc when cutting a release corresponding to a new server version.

It seems logical that a 4.2-compatible release of Motor would require the 4.2-compatible PyMongo release at a minimum. However, with the way things are currently setup, the 4.2-compatible release of Motor would install with any 4>PyMongo>=3.6. This will result in unexpected errors if the user attempts to use server features that aren't supported by the version of PyMongo installed. More confusingly still, such a scheme decouples server-compatbility from the Motor-version in use, as users need to inspect the PyMongo version installed in order to infer the latest supported server version.