Python3: use six.StringIO rather than StringIO.StringIO
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

alexpilotti/python-glanceclient,klmitch/python-glanceclient,mmasaki/python-glanceclient,alexpilotti/python-glanceclient,openstack/python-glanceclient,varunarya10/python-glanceclient,JioCloud/python-glanceclient,JioCloud/python-glanceclient,openstack/python-glanceclient,mmasaki/python-glanceclient,klmitch/python-glanceclient,varunarya10/python-glanceclient