I'm not sure __path__ is mandatory anymore, as I believe it's just a way to override __spec__. I'll double check the importlib code.

Edit: after checking the importlib code, I'm happy simply looking for __path__ is still the right check for runtime introspection. importlib itself handles merging __path__, __spec__.is_package and __spec__.submodule_search_locations into a single final __path__ list when it creates the import package module instances.

This also keeps things consistent with the glossary's definition of a package as a module with a __path__ attribute.