Pymongo uses a different implicit session for each cursor returned from the parallelCollectionScan command. This worked fine in 3.6 but in 4.0 the getMore with a different lsid fails with:
OperationFailure: Cannot run getMore on cursor 625787356667, which was created in session fa0f13b5-db90-49fa-9d93-f0633db9289a - 47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=, in session 6b6390df-a5af-4b56-b942-683a2ee9cf02 - 47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

Depending on the server's recommendation in SERVER-34417, we may need to use the same session for the initial parallelCollectionScan command and all the resulting cursors. This may involve reference counting the session.