archivers/py-borgbackup: security upgrade to 1.2.6

1.2.5 has been skipped for FreeBSD ports because the necessary upgrade
procedure did not work for all archives.

Changelog: https://github.com/borgbackup/borg/blob/1.2.6/docs/changes.rst#version-126-2023-08-31
Security: https://github.com/borgbackup/borg/blob/1.2.6/docs/changes.rst#pre-125-archives-spoofing-vulnerability-cve-2023-36811
Security: CVE-2023-36811
Security: b8a52e5a-483d-11ee-971d-3df00e0f9020
MFH: 2023Q3 after some stabilization time