Partial fix of test_strutils.py on Python 3

StringToBytesTest does not work on Python 3 because it uses testscenarios which
does not work in nosetests, see the bug:
https://bugs.launchpad.net/testscenarios/+bug/872887

Because of this bug, the test_strutils.py is not enabled yet for py33 in
tox.ini.

Change-Id: Ie3a7742dfd802f21c6c7420e3b894122e1dca295
Merge "Partial fix of test_strutils.py on Python 3"