Fix MP3 tag data writes

[BenjaminPelletier]

Currently, Mutagen drastically increases the file size of MP3 files when new tags are written. To reproduce, something like:

import os

import mutagen.id3

filename = '01 - Vendetta.mp3'
print('Mutagen %s' % mutagen.version_string)
print('Before writing tag: %d' % os.path.getsize(filename))
id3 = mutagen.id3.ID3(filename)
frame = mutagen.id3.TXXX(desc='StreamTagger: Tags', text=['{"letsdance": {"ben": {"t": "2019-11-08T04:19:01", "v": 3}}}'])
id3.add(frame)
id3.save(filename=filename, v2_version=3)
print('After writing tag: %d' % os.path.getsize(filename))