Merge pull request #7367 plus own fixes.

This introduces beets 1.3.11, changes can be found at:

https://github.com/sampsyo/beets/releases/tag/v1.3.11

Thanks to @pjones for the pull request, which also adds him to the
maintainers.

My changes on the other hand are only of cosmetic nature and do not
change anything in functionality.