bpo-41203: Replace Mac OS X and OS X with macOS (GH-28515) (GH-28523)

Replace old names when they refer to actual versions of macOS.
Keep historical names in references to older versions.

Co-authored-by: Patrick Reader <_@pxeger.com>
(cherry picked from commit 36122e18148c5b6c78ebce1d36d514fd7cf250f5)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>