python3Packages.exceptiongroup: adjust dependencies for Python<=3.12 (#413990)

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

···
   pytestCheckHook,
   pythonAtLeast,
   pythonOlder,
+  typing-extensions,
 }:
 
 buildPythonPackage rec {
···
     hash = "sha256-b3Z1NsYKp0CecUq8kaC/j3xR/ZZHDIw4MhUeadizz88=";
   };
 
-  nativeBuildInputs = [ flit-scm ];
+  build-system = [ flit-scm ];
+
+  dependencies = lib.optionals (pythonOlder "3.13") [ typing-extensions ];
 
   nativeCheckInputs = [ pytestCheckHook ];