I now have commit access to the main mutagen repository, so I'm currently porting the changes I've made here across to the original project. Hopefully, soon I can start focusing on some of the other improvements I have planned apart from Python 3 support.
mutagenx was originally a project to create a Python 3 library which implemented all the features of the Python 2 library mutagen. After the developer of mutagen made some visible progress in making the mutagen code Python 3 compatible, I decided to do the same, in the other direction. So, we now have two projects, moving together from opposite sides of the problem, aiming to achieve a unified codebase.
Once mutagen supports Python 3, I intend to continue with the mutagenx project, adding various features I think should be in mutagen. I hope it will act as a testing ground for things that end up going into mutagen. The first of these improvements will be a common interface for accessing metadata, to replace the EasyMP3 and EasyMP4 classes with a system which works for all formats.
This should greatly simplify the majority of metadata editing tasks, and make it unnecessary to use the format-specific metadata classes. It will also enable projects such as MusicBrainz Picard to scrap a ton of metadata specific code.
Please see the README.rst file for the current README included in mutagen.