At http://api.mongodb.org/python/current/api/pymongo/mongo_replica_set_client.html#pymongo.mongo_replica_set_client.MongoReplicaSetClient, the example call is:

class pymongo.mongo_replica_set_client.MongoReplicaSetClient([hosts_or_uri[, max_pool_size=10[, document_class=dict[, tz_aware=False[, **kwargs]]]]])

This is missing the required replicaSet argument. It's mentioned in the following text, but should also be in the example.