anchor.diff:  , enableGtk3 ? false, cairo
   # darwin has its own "MacOSX" backend
   , enableTk ? !stdenv.isDarwin, tcl ? null, tk ? null, tkinter ? null, libX11 ? null
  -, enableQt ? false, pyqt4
  +, enableQt ? false, pyqt5 ? null
   , libcxx
   , Cocoa
   , pythonOlder
  
positive.code:                 && (tkinter != null)
                  && (libX11 != null)
                  ;
  assert enableQt -> pyqt4 != null;
  
  buildPythonPackage rec {
    version = "3.1.1";
  
negative.code:     ++ stdenv.lib.optional enableGtk2 pygtk
      ++ stdenv.lib.optionals enableGtk3 [ cairo pycairo gtk3 gobject-introspection pygobject3 ]
      ++ stdenv.lib.optionals enableTk [ tcl tk tkinter libX11 ]
      ++ stdenv.lib.optionals enableQt [ pyqt5 ];
  
    patches =
      [ ./basedirlist.patch ];
  