Add a package for py-borg, aka Borg Backup. It is a successor of
sysutils/py-attic, which is no longer being developed upstream.

BorgBackup (short: Borg) is a deduplicating backup program. Optionally,
it supports compression and authenticated encryption.

The main goal of Borg is to provide an efficient and secure way to
backup data. The data deduplication technique used makes Borg suitable
for daily backups since only changes are stored. The authenticated
encryption technique makes it suitable for backups to not fully trusted
targets.

(bsiegert)