Python3: use six.StringIO rather than StringIO.StringIO

It’s an alias for StringIO.StringIO in Python 2 and
io.StringIO in Python 3.

Change-Id: I1c2f1412c586d5d8e0aa866a28232747056e0ca7
Python3: use six.StringIO rather than StringIO.StringIO

It’s an alias for StringIO.StringIO in Python 2 and
io.StringIO in Python 3.

Change-Id: I5316eaffa2d9d2d5679b85a901933ef0fbfcc2f7
Merge "Python3: use six.StringIO rather than StringIO.StringIO"
Merge "Python3: use six.StringIO rather than StringIO.StringIO"