bpo-44025: Clarify when '_' is a keyword. (GH-25873)

In match statements, in case patterns and nowhere else.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 3b200b2aa648fcc8a2673871807c1463afe00195)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>