bpo-43795: Don't list private names in the limited API (GH-26740)

* Remove struct _node from the stable ABI list

This struct was removed along with the old parser in Python 3.9 (PEP 617)

* Stable ABI list: Use the public name "PyFrameObject" rather than "_frame"

* Ensure limited API doesn't contain private names

Names prefixed by an underscore are private by definition.

* Add a blurb