Removes python3 and PyPy support

Over the last few days its become clear to me that the Qpid Transport for Kombu cannot adequately support Python3 and PyPy at this time. The upstream test runner (Travis) tests those environments. This commit does four things:

1. Adjusts the test behavior to have all tests skip in Python 3 and PyPy environments. Using this code it passes in Travis and the tests do all run using Python 2.7. They also work in Python 2.6 so that our building of kombu in Python 2.6 environments still works. This causes upstream Travis to pass for all environments!
2. Updates the Kombu docs to indicate that the Qpid Transport support Python 2.7, but not Python3 or PyPy
3. Adds runtime checks to ensure that Python3 and PyPy environments are not being used.
4. Adds test coverage for the runtime checks that were added. Test coverage is still 100%
Fixed with a1439661059c78469948c2831681ebb697ae5700 and also updated the module docstring at the top similarly.

You identified why Python 2.6 is not stated even though it does work. We know this runs in Python 2.6, but that is a downstream concern.