Add shell.nix

shell.nix

@@ -0,0 +1,22 @@
+let
+  mach-nix = import
+    (builtins.fetchGit {
+      url = "https://github.com/DavHau/mach-nix";
+      ref = "refs/tags/3.5.0";
+    })
+    { };
+
+  requirements = builtins.readFile ./requirements.txt;
+
+  requirements-dev = ''
+    django-debug-toolbar
+    ipython
+    black
+    isort
+    flake8
+  '';
+in
+
+mach-nix.mkPythonShell {
+  requirements = requirements + requirements-dev;
+}