[Borgbackup] bundling msgpack for borgback distribution

[Thomas Waldmann]

> Anyway, I definitely got a
> sense of deja vu seeing those errors, as well as seeing the patch at
> https://github.com/borgbackup/borg/commit/0ebfaa5b61a675c22cda301bc20d0b00372dd181

That patch is just for borg master branch, which already has been
adapted before to work with recent msgpack versions.

If you dig in issue tracker and git history, you'll see a lot of msgpack
related changes in master branch (but not in 1.1-maint branch).

msgpack releases are of quite varying quality, there has been a lot of
buggy releases a while ago. also there are sometimes problematic api
changes.

otoh, having a working msgpack is quite essential for borg working
correctly and for people having working backups and restores.

this is why i got rather picky about msgpack releases and why the
requirements in borg's setup.py are that strict and new msgpack releases
only get added there after some testing.

> (My current solution is to downgrade msgpack, since I found nothing else
> was using it; credit to
> https://bugzilla.redhat.com/show_bug.cgi?id=1669083#c11 for this)

The question here is:
- is nothing else requiring the newest msgpack ON YOUR SYSTEM
- is nothing else requiring it considering all existing dist packages

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