Update dependency to latest mutagen version

Mutagen 0.20 has a bug when querying bitrate of Musepack files. Let's make life
easier on everyone and make sure they have up to date version. As a bonus
mediafile tests don't fail with 0.21
Fix python namespaces for test runs

We need to make sure we don't use namespaced versions that are already installed
on the system but rather use local version from current sources
Merge pull request #245 from sochotnicky/master

Update mutagen dependency to 0.21