luigi: remove dependency override on tornado_5

When building luigi on python3, upstream supports the latest tornado

pkgs/applications/networking/cluster/luigi/default.nix

@@ -1,22 +1,15 @@
 { lib, python3 }:
 
-let
-  python = python3.override {
-    self = python;
-    packageOverrides = self: super: {
-      tornado = super.tornado_5;
-    };
-  };
-in with python.pkgs; buildPythonApplication rec {
+python3.pkgs.buildPythonApplication rec {
   pname = "luigi";
   version = "3.0.2";
 
-  src = fetchPypi {
+  src = python3.pkgs.fetchPypi {
     inherit pname version;
     sha256 = "b4b1ccf086586d041d7e91e68515d495c550f30e4d179d63863fea9ccdbb78eb";
   };
 
-  propagatedBuildInputs = [ dateutil tornado_5 python-daemon boto3 ];
+  propagatedBuildInputs = with python3.pkgs; [ dateutil tornado python-daemon boto3 ];
 
   # Requires tox, hadoop, and google cloud
   doCheck = false;

pkgs/top-level/python-packages.nix

@@ -7772,7 +7772,7 @@ in {
   # Used by circus and grab-site, 2020-08-29
   tornado_4 = callPackage ../development/python-modules/tornado/4.nix { };
 
-  # Used by luigi, 2020-08-29
+  # Used by streamlit, graphite_beacon, 2021-01-29
   tornado_5 = callPackage ../development/python-modules/tornado/5.nix { };
 
   towncrier = callPackage ../development/python-modules/towncrier { inherit (pkgs) git; };