pythonPackages.kombu: Enable for python > 2.7

Only tests cannot run on python3* because dependencies are unavailable
(qpid-python). Kombu is announced to work fine for python3[1]

[1] https://github.com/celery/kombu/blob/a4edc670f49027154e3f708dfcd3726aceda8fc5/setup.py#L168-L170

pkgs/top-level/python-packages.nix

@@ -12137,13 +12137,13 @@ in modules // {
     };
 
     # most of these are simply to allow the test suite to do its job
-    buildInputs = with self; [ mock unittest2 nose redis qpid-python pymongo sqlalchemy pyyaml msgpack boto ];
+    buildInputs = with self; optionals isPy27 [ mock unittest2 nose redis qpid-python pymongo sqlalchemy pyyaml msgpack boto ];
 
     propagatedBuildInputs = with self; [ amqp anyjson ] ++
       (optionals (pythonOlder "2.7") [ importlib ordereddict ]);
 
     # tests broken on python 2.6? https://github.com/nose-devs/nose/issues/806
-    doCheck = (pythonAtLeast "2.7");
+    doCheck = isPy27;
 
     meta = {
       description = "Messaging library for Python";