archivers/py-borgbackup12: update to 1.2.9
Which is a bugfix release.
Changelog: https://github.com/borgbackup/borg/blob/1.2.9/docs/changes.rst#version-129-2025-08-04
MFH: 2025Q3 after a few days
(cherry picked from commit e17c8ec3c71c1ed9193ad914ae9eb7773331be48)