OK, so the warning is triggered if __package is None or __spec__ is None (https://github.com/python/cpython/blob/ac5bbd43bc7b769c13ae0412cb28a3521f4d4ff1/Lib/importlib/_bootstrap.py#L1038). That's defined in _calc___package__() which is only called if index != 0 when calling __import__() (https://github.com/python/cpython/blob/ac5bbd43bc7b769c13ae0412cb28a3521f4d4ff1/Lib/importlib/_bootstrap.py#L1062 or https://github.com/python/cpython/blob/ac5bbd43bc7b769c13ae0412cb28a3521f4d4ff1/Python/import.c#L1520).

So the question becomes how is Cython importing modules? This warning should only be triggered if you're attempting a relative import from within a package (and thus have a level > 0). Based on Antoine's Cython issue, since something like multidict isn't a package it would suggest either Cython is setting the level > 0 when it doesn't mean to or there's a bug somewhere with level == 0 and yet Python is still trying to calculate the parent package for no reason.