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