add platformdirs python package to flake.nix

This is required (but not automatically pulled in) by the black
pre-commit hook.

Author: lilioid

flake.nix

@@ -22,6 +22,7 @@
           packages = with pkgs; [
             python312
             python312Packages.ipython
+            python312Packages.platformdirs
             uv
           ];
         };