The definition of __str__ is different between py2 and py3.
__str__ should return a byte stream to make print(exc) work
in python2, but __str__ should return string type in python3.
six.python_2_unicode_compatible sets up __str__ and __unicode__
approriately.
Change-Id: I6c85ca8c4e6f86450ee390db81e3aa101293b846
Related-Bug: #1235228
