This was removed because I thought it's for Python<3 but it's for Python<3.7.

* flake8: noqa the Python 2-only builtin cmp()

* flake8: buffer() was removed in Python 3

* flake8: unicode() was removed in Python3 because all str are Unicode

Adds the modifications from #1227

* buffer() was removed in Python 3 so use memoryview

https://docs.python.org/2.7/library/stdtypes.html#memoryview-type

* buffer = bytes on Python 3

* from kombu.five import buffer

* buffer --> buffer_t

* @patch('buffer_t') in test_qpid.py

* @patch('kombu.five.buffer_t')

* Use buffer on legacy Python and bytes on Python

* Update test_qpid.py