six is the canonical compatibility library for supporting Python 2 and 3
in a single codebase.
The basestring was removed in Python 3 and we should use 'six.string_types'
instead of 'basestring' to make code compatible with py 2 and 3 as well.
Partially-implements blueprint py3-compatibility
Change-Id: Idb452afdc2a828089627e14f90c53f3967ceb2ad