shell.nix: Update to use virtualenv

shell.nix

@@ -1,22 +1,19 @@
-let
-  mach-nix = import
-    (builtins.fetchGit {
-      url = "https://github.com/DavHau/mach-nix";
-      ref = "refs/tags/3.5.0";
-    })
-    { };
+{ pkgs ? import <nixpkgs> { }, ... }:
 
-  requirements = builtins.readFile ./requirements.txt;
+pkgs.mkShell {
+  shellHook = ''
+    export DJANGO_SETTINGS_MODULE=gestioasso.settings.local
 
-  requirements-dev = ''
-    django-debug-toolbar
-    ipython
-    black
-    isort
-    flake8
+    virtualenv .venv
+    source .venv/bin/activate
   '';
-in
 
-mach-nix.mkPythonShell {
-  requirements = requirements + requirements-dev;
+  packages = with pkgs; [
+    python38
+    python38Packages.pip
+    python38Packages.virtualenv
+    python38Packages.python-ldap
+  ];
+
+  allowSubstitutes = false;
 }