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 https://github.com/NixOS/nixpkgs/issues/253154
cc @mweinelt @figsoda

pkgs/development/python-modules/agate-sql/default.nix

@@ -12,6 +12,7 @@
 buildPythonPackage rec {
   pname = "agate-sql";
   version = "0.7.0";
+  format = "setuptools";
 
   disabled = isPy27;