libsass: 3.5.2 -> 3.5.3

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

This update was made based on information from https://repology.org/metapackage/libsass/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 3.5.3 with grep in /nix/store/mp09kp6kk1mnb0m6szp8r8cjswpky236-libsass-3.5.3
- directory tree listing: https://gist.github.com/693ae2149cf8d174efe563ad97396c0a