add nix venv

default.nix

@@ -0,0 +1,20 @@
+with import <nixpkgs> {};
+
+stdenv.mkDerivation {
+  name = "myPythonEnv";
+  buildInputs = with pkgs; [
+    python37Full
+    python37Packages.virtualenv
+  ];
+  src = null;
+  shellHook = ''
+    if [ ! -d .venv ]; then
+      python -m venv .venv
+    fi
+    source .venv/bin/activate
+    pip install --upgrade pip
+    if [ -s requirements.txt ]; then
+      pip install -r requirements.txt
+    fi
+  '';
+}

requirements.txt

@@ -0,0 +1,3 @@
+pillow
+wheel
+colour