util: add shell.nix

shell.nix

@@ -0,0 +1,11 @@
+{ pkgs ? import <nixpkgs> {} }:
+with pkgs; mkShell rec {
+  buildInputs = [
+    (python3.withPackages (ps: with ps; [
+      flask
+      flask_assets
+      libsass
+      setuptools
+    ]))
+  ];
+}