>> Can you test the python code from python-shell-completion-native-setup
>> outside of Emacs, e.g., save it to a file called 'native-completion.py'
>> and then run 'python -i native-completion.py' and then type an
>> underscore and hit <tab>.
>
> I get the following:
>
>>>> ___package__
> __PYTHON_EL_native_completion_setup
> __name__
> __doc__
> __import__
> __debug__
>
> Interestingly, none of the dummy completions pop up.

This seems to be the core of the problem.  I gather that macOS uses
libedit instead of readline by default, perhaps that is the source of
incompatibility.  Can you figure out how to change the python code so
that the dummy completions do show up?