tuir: unbreak adding six dependency

pkgs/applications/misc/tuir/default.nix

···
 
   checkInputs = [ coverage coveralls docopt mock pylint pytest vcrpy ];
 
-  propagatedBuildInputs = [ beautifulsoup4 decorator kitchen requests ];
+  propagatedBuildInputs = [ beautifulsoup4 decorator kitchen requests six ];
 
   meta = with lib; {
     description = "Browse Reddit from your Terminal (fork of rtv)";