pythonix: mark it as a python module

This was mentioned by @FRidh in https://github.com/NixOS/nixpkgs/pull/33174#discussion_r159083763

pkgs/top-level/python-packages.nix

···
     };
   };
 
-  pythonix = callPackage ../development/python-modules/pythonix { };
+  pythonix = toPythonModule (callPackage ../development/python-modules/pythonix { });
 
   pypolicyd-spf = buildPythonPackage rec {
     name = "pypolicyd-spf-${version}";