beets 1.1.0 has been released. renaming the ebuild worked for me. The installation should have failed, because I had only media-libs/mutagen-1.20 installed, but it worked fine. emerge should have requested 1.21. Not sure if it's a general problem or just some oddity on my side. Reproducible: Always
Beets updated to 1.1.0 (1.2.0 ready, just waiting on discogs library - bug 472408 if you want to keep track). I also migrated most of beets dependencies to new EAPI 5 so that should solve the mutagen problems.