Add id3 TSO2 tag to albumartist_sort

This tag is a non-standard tag used by iTunes, Apple Music, MusicBrainz
Picard, and Swinsian (at least) in preference of the User Frame ALBUMARTISTSORT.

See the Picard Tag Mapping documentation at
https://picard-docs.musicbrainz.org/downloads/MusicBrainz_Picard_Tag_Map.html

By adding this I'm able to get beets to update the Album Artist Sort in
Swinsian.

Thanks to James Bruton who develops Swinsian for the help diagnosing why
my fields weren't changing in Swinsian, and pointing me at the Picard
documentation showing its preference for TSO2.

# Conflicts:
#	docs/changelog.rst
#	mediafile.py

Author: telyn

docs/changelog.rst

@@ -14,6 +14,8 @@ v1.0.0
 - Moved changelog into its own file, ``changelog.rst``. Also added github
   workflow for automatic changelog reminders.
 - Modernized package and tests setup to use ``poetry``.
+- Added TSO2 tag to ``albumartist_sort``, matching how Picard >= 1.2, iTunes and
+    Swinsian interpret tags.
 
 v0.13.0
 -------

mediafile/__init__.py

@@ -514,6 +514,7 @@ def as_dict(self):
         ASFStorageStyle("WM/ArtistSortOrder"),
     )
     albumartist_sort = MediaField(
+        MP3StorageStyle("TSO2"),
         MP3DescStorageStyle("ALBUMARTISTSORT"),
         MP4StorageStyle("soaa"),
         StorageStyle("ALBUMARTISTSORT"),