py3: Use six module for StringIO imports

It is python 2/3 compatibility change.
six.StringIO an alias for StringIO.StringIO in Python 2 and
io.StringIO in Python 3.

Partially implements blueprint py3-compatibility

Change-Id: Ib2f570645d3b0a4b3bd38c017693e0b1b77f3236
Merge "py3: Use six module for StringIO imports"