python.pkgs.traitlets: fix build and don't include .pytest_cache

Fixes build and prevents including `.pytest_cache` file (#40259).

pkgs/development/python-modules/traitlets/default.nix

@@ -7,6 +7,8 @@
 , ipython_genutils
 , decorator
 , enum34
+, pythonOlder
+, six
 }:
 
 buildPythonPackage rec {
@@ -20,12 +22,14 @@ buildPythonPackage rec {
   };
 
   checkInputs = [ glibcLocales pytest mock ];
-  propagatedBuildInputs = [ ipython_genutils decorator enum34 ];
+  propagatedBuildInputs = [ ipython_genutils decorator six ] ++ lib.optional (pythonOlder "3.4") enum34;
 
   checkPhase = ''
-    LC_ALL="en_US.UTF-8" py.test $out
+    LC_ALL="en_US.UTF-8" py.test
   '';
 
+#   doCheck = false;
+
   meta = {
     description = "Traitlets Python config system";
     homepage = http://ipython.org/;