.github/workflows: update Python build versions

Drop Python 3.8.
Add Python 3.13.

Drop macOS 12.
Add macOS 15.

Change-Id: I5916cf2de54dfb0274e50a8db3b8d93a60940710
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63730
Reviewed-by: Alan Donovan <adonovan@google.com>