Added MP3DescStorageStyle for LABEL and MEDIA (#95)

This PR adds the `TXXX:LABEL` and `TXXX:MEDIA` to the respective fields
in the MediaFile class.

These fields are read and written by foobar and Mp3Tag.

For references see

https://community.mp3tag.de/t/the-case-for-a-dedicated-label-tag/57647/17
and
https://community.mp3tag.de/t/what-is-the-correct-tag-for-label/59445/2

closes #85

Author: semohr

docs/changelog.rst

@@ -22,6 +22,8 @@ v1.0.0
 - 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.
+- Added ``TXXX:LABEL`` and ``TXXX:MEDIA`` tags to ``label`` and ``media``
+  fields, respectively, for MP3 files.
 
 v0.13.0
 -------

mediafile/__init__.py

@@ -496,6 +496,7 @@ def as_dict(self):
 
     label = MediaField(
         MP3StorageStyle("TPUB"),
+        MP3DescStorageStyle("LABEL"),
         MP4StorageStyle("----:com.apple.iTunes:LABEL"),
         MP4StorageStyle("----:com.apple.iTunes:publisher"),
         MP4StorageStyle("----:com.apple.iTunes:Label", read_only=True),
@@ -596,6 +597,7 @@ def as_dict(self):
     )
     media = MediaField(
         MP3StorageStyle("TMED"),
+        MP3DescStorageStyle("MEDIA"),
         MP4StorageStyle("----:com.apple.iTunes:MEDIA"),
         StorageStyle("MEDIA"),
         ASFStorageStyle("WM/Media"),