mopidy-iris: 3.17.5 -> 3.18.0

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
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 3.18.0 with grep in /nix/store/nldsjybf1v0fbjnbfgcknykr4q6fjq9c-Mopidy-Iris-3.18.0
- directory tree listing: https://gist.github.com/0ad20de473538813e443e421603c7997