flask: depend on setuptools

The `flask` CLI uses `setuptools` (or more specifically `pkg_resources`) [here](fdac8a5404/src/flask/cli.py (L498)) to discover third party commands, and just does nothing silently if it's not found. Add the dependency so custom commands start working.

pkgs/development/python-modules/flask/default.nix

@@ -7,6 +7,7 @@
 , jinja2
 , python-dotenv
 , werkzeug
+, setuptools
 , pytestCheckHook
 }:
 
@@ -26,6 +27,10 @@ buildPythonPackage rec {
     itsdangerous
     jinja2
     werkzeug
+
+    # required for CLI subcommand autodiscovery
+    # see: https://github.com/pallets/flask/blob/fdac8a5404e3e3a316568107a293f134707c75bb/src/flask/cli.py#L498
+    setuptools
   ];
 
   checkInputs = [