The most likely relevant difference here is that Python 3.4+ no longer forcibly break cycles through the module globals when the module is deallocated: https://docs.python.org/dev/whatsnew/3.4.html#whatsnew-pep-442

Due to the implicit cycles created between function definitions and their global namespace via the __globals__ attribute on the function, this means that embedding applications will need to explicitly run a GC collection cycle after deleting a module in order to fully finalise it.