bpo-43532: add version added to KW_ONLY (GH-31235)

Minor missing version note mentioned at the end of  (and affected me independently before reading that note).

Automerge-Triggered-By: GH:ericvsmith
(cherry picked from commit 5a3f97291eea96037cceee097ebc00bba44bc9ed)

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>