mopidy-iris: 3.18.0 -> 3.20.0 (#41534)

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 3.20.0 with grep in /nix/store/0v9g0y8ik2ywqz9yn4znad73la2g0545-Mopidy-Iris-3.20.0
- directory tree listing: https://gist.github.com/5b11f78cc0e1e8f1665c9951f6b1b9d0
- du listing: https://gist.github.com/7bd8ef1ac69ff6560db53865c6a9e322