You're right, "impossible" is a slight exaggeration :-). As an alternative, every package could indeed carry around a table containing the details of importlib's call stack in every version of Python.

(I also haven't checked whether it's consistent within a single stable release series. I guess we could add a rule that 3.5.x -> 3.5.(x+1) cannot change the number of function calls inside the importlib callstack, because that is part of the public API, but I have less than perfect confidence that this rule has been enforced strictly in the past, and I don't think I'd want to be the one in charge of enforcing it in the future.)