I think when we support python >=3.7, we’ll have the hooks necessary to actually warn about this.

It's actually not much harder to emit a warning on Py3.6 (#10735); actually I don't like the module-level __getattr__ addtion in Py3.7 at all :/ as the tools to do that were already present, and the module-level __getattr__ introduces a weird exception to the rule that dunders are looked up on the class (here that would be ModuleType), not on the instance (the module itself).