Use six.moves.StringIO instead of StringIO.StringIO

In Python 3, StringIO module is gone.
Class StringIO.StringIO is not available from io module.
To support both Python 2 and Python 3, "six.moves.StringIO" should be used instead.

Change-Id: I17e79d8625c75d6903f024c648d3ef62e21d0c51
Closes-bug:#1403076