move default.nix to shell.nix

python_environment/shell.nix

@@ -0,0 +1,18 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  name = "python-environment";
+  buildInputs = with pkgs; [
+    python3
+    python3Packages.virtualenv
+  ];
+  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
+  '';
+}