This looks like https://github.com/beetbox/beets/issues/2153 i.e. that the
current version of mutagen that we have in Debian (1.36) is incompatible with
the version of beets (1.3.19). Quoting from that issue:
> when beets 1.3.19 was released, Mutagen 1.33 didn't exist yet.
> When that was released, it changed the way exceptions work, which
> broke those tests. We've since fixed compatibility with the
> latest version, but we can't go back in time and change the
> version specifier for 1.3.19.
So the easiest way to fix this will be to update to the latest upstream beets in
Debian.
