mopidy-iris: 3.20.0 -> 3.21.1

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/Mopidy-Iris/versions.

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- found 3.21.1 with grep in /nix/store/ydhbxhldy8a9n64r8if9zy4ricl4in20-Mopidy-Iris-3.21.1
- directory tree listing: https://gist.github.com/3ad4fbaeff603abf3078d8371547dbb4
- du listing: https://gist.github.com/bcb4096a10cef861d8bcf1f384c7ae3e