In links: http://pythonhosted.org/six/, we know 'cStringIO' is
a module of python 2.x, but in python3.x module 'io' has the
same function as 'cStringIO' in python2.x. So here we use
six.stringIO instead.
TrivialFix.
Change-Id: I8022b92b186e239cd76a36dab7766301bf96d2c4