Fix beets.mediafile deprecation warning

[labrys]

Description

Fixes #1958

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have based this change on the nightly branch
• [x] I have performed a self-review of my own code
• [x] New and existing unit tests pass locally with my changes

[labrys]

Going to wait on this till Python 2.7 is dropped.