pythonPackages.flask: Move optional deps to passthru

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

@@ -28,8 +28,6 @@ buildPythonPackage rec {
   };
 
   propagatedBuildInputs = [
-    asgiref
-    python-dotenv
     click
     itsdangerous
     jinja2
@@ -43,6 +41,10 @@ buildPythonPackage rec {
   passthru.tests = {
     inherit flask-limiter flask-restful flask-restx moto;
   };
+  passthru.optional-dependencies = {
+    dotenv = [ python-dotenv ];
+    async = [ asgiref ];
+  };
 
   meta = with lib; {
     homepage = "https://flask.palletsprojects.com/";