Regarding the problem with the default value, can we use “unspecified”, as documented at <https://docs.python.org/dev/howto/clinic.html#writing-a-custom-converter>, or is that an old relic that no longer exists (hinted at the end of Issue 20293)?
Failing that, I prefer Raymond’s earlier suggestion: say that any support for −1 or None are implementation details, and not part of the API. The same statement could be added to the native Python doc strings as well.
BTW I am not opposed to the proposal for proper hi=None support either, although it does complicate the change. IMO allowing hi=None could be done as a second step.