Adds the following fields with id3v2.4 multi-valued tag support to autotag:
- artists, artists_sort, artists_credit
- albumartists, albumartists_sort, albumartists_credit
- mb_artistids, mb_albumartistids
MusicBrainz support to populate + write the above multi-valued tags by default. Can be toggled to use id3v2.3 or id3v2.4 tags via the existing beets configuration option `id3v23`.
Big thanks to @JOJ0, @OxygenCobalt, @arsaboo for testing + @sampsyo for the initial code review .
these are mostly in the tests, which didn't cause issues since the
affected directories usually have nice ASCII paths. For consistency, it
is nicer to always invoke syspath. That also avoids deprecation warnings
for the bytestring interfaces on Python <= 3.5. The bytestring
interfaces were undeprecated with PEP 529 in Python 3.6, such that we
didn't observe any actual failures.
