In MongoClient's docstring:

If connecting directly to a secondary then a read preference mode other than PRIMARY is required - otherwise all queries will throw AutoReconnect “not master”.

This is no longer the case, we obey the Server Selection Spec:

Topology type Single: clients MUST always set the slaveOK wire protocol flag on reads to ensure that any server type can handle the request.

https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#topology-type-single