python3Packages.visitor: migrate to pyproject

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

@@ -2,18 +2,21 @@
   lib,
   buildPythonPackage,
   fetchPypi,
+  setuptools,
 }:
 
 buildPythonPackage rec {
   pname = "visitor";
   version = "0.1.3";
-  format = "setuptools";
+  pyproject = true;
 
   src = fetchPypi {
     inherit pname version;
     sha256 = "02j87v93c50gz68gbgclmbqjcwcr7g7zgvk7c6y4x1mnn81pjwrc";
   };
 
+  build-system = [ setuptools ];
+
   meta = {
     homepage = "https://github.com/mbr/visitor";
     description = "Tiny pythonic visitor implementation";