python3.pkgs.*: Explicitly pass buildPythonPackage format parameter

Long term we should move everything over to `pyproject = true`, but in
the mean time we can work towards deprecating the implicit `format` paremeter.

cc NixOS/nixpkgs#253154
cc @mweinelt @figsoda

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

@@ -9,6 +9,7 @@
 buildPythonPackage rec {
   pname = "vdf";
   version = "3.4";
+  format = "setuptools";
 
   src = fetchFromGitHub {
     owner = "ValvePython";