beets: 1.4.6 -> 1.4.7

The upstream release notes are a bit bigger, so I'm not including it
here. You can find it at:

https://github.com/beetbox/beets/releases/tag/v1.4.7

Originally I wanted to just fix the build, which is currently broken
because a few tests have failed, but the fix for the tests is already
included in the 1.4.7 upstream release[1], so I opted to update instead.

Other than running the tests included in beets (in addition to
building/running tests of the "alternatives" and "copyartifacts"
external plugins) I have made some small queries on my local music
collection, but haven't tested import or any write operation.

[1]: https://github.com/beetbox/beets/commit/8eb50fee33044dea008408423ee



Signed-off-by: aszlig <aszlig@nix.build>
Cc: @domenkozar, @pjones