topydo: use throw instead of trace to not break evaluation

pkgs/top-level/python-packages.nix

···
     };
   };
 
-  topydo = builtins.trace "python3Packages.topydo was moved to topydo" pkgs.topydo; # 2017-09-22
+  topydo = throw "python3Packages.topydo was moved to topydo"; # 2017-09-22
 
   w3lib = buildPythonPackage rec {
     name = "w3lib-${version}";