Use io.StringIO to replace StringIO.StringIO, and use unicode as StringIO()
can only handle unicode strings.
Story: 2003429
Task: 24618
Change-Id: Ie2ced7bba00d11b1698422cbe495abc0fbab7637
Signed-off-by: chenyan <yan.chen@intel.com>
Signed-off-by: Sun Austin <austin.sun@intel.com>