mopidy-iris: 3.21.1 -> 3.21.3 (#42474)

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 3.21.3 with grep in /nix/store/qb6067h8y8p8y7kqkmdk308a494milyw-Mopidy-Iris-3.21.3
- directory tree listing: https://gist.github.com/e0c3564e49b2cec4b30d98ec42723bff
- du listing: https://gist.github.com/a127f023f22f9a2abd69c2dd0ff0c71b