Add id3 TSO2 tag to albumartist_sort (#91)

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

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!

I believe this fixes #90.

Author: semohr

docs/changelog.rst

@@ -20,6 +20,8 @@ v1.0.0
   workflow for automatic changelog reminders.
 - Modernized package and tests setup to use ``poetry``.
 - Run pyupgrade to align code with Python 3.10+ syntax.
+- Added TSO2 tag to ``albumartist_sort``, matching how Picard >= 1.2, iTunes and
+      Swinsian interpret tags.
 
 v0.13.0
 -------

mediafile/__init__.py

@@ -510,6 +510,7 @@ def as_dict(self):
         ASFStorageStyle("WM/ArtistSortOrder"),
     )
     albumartist_sort = MediaField(
+        MP3StorageStyle("TSO2"),
         MP3DescStorageStyle("ALBUMARTISTSORT"),
         MP4StorageStyle("soaa"),
         StorageStyle("ALBUMARTISTSORT"),