Fix HdmiCecControl on Python 2
Fix HdmiCecControl on Python 2

This is part-way ported to Python 3 (e.g. we're using unicode string
literals by default). This commit undoes some of the automatic
changes made by `futurize` and fixes some unicode/bytes issues.

What's still missing for Python 3:

- We're still using Python 2's builtin `str` because python-future's
  backported implementation of `builtins.bytes` doesn't work if given a
  `buffer` of bytes.

- We're still using `buffer` (instead of `memoryview`) because Python
  2's ctypes doesn't work with Python 2's memoryview:
  https://stackoverflow.com/questions/28984692#comment46219618_28984692

Note that HdmiCecControl will fail *anyway* with Python 3 because
there's no python3-libcec on Ubuntu 18.04. So there's no urgency to fix
the above points.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester