In python2 this was easy to support you just defined the __dir__ property.
But that was removed in python3 and there is no direct replacement.

What makes this work for python3 is using an API to create classes that I could not see how
to use in PyCXX. (I don't have the API details to hand at the moment).

Curiously this has been this way with PyCXX since I added python 3.9 support and your maybe the 2nd person to notice.

I would love to fix this, but have not figured out how.