Use six.StringIO/BytesIO instead of StringIO.StringIO
Use six.StringIO/BytesIO instead of StringIO.StringIO

to keep Python 3.x compatibility, use six.StringIO/BytesIO to
replace StringIO.StringIO

StringIO works for unicode
BytesIO works for bytes

Change-Id: Id7f4c3f701275229714f45296eac9f7f34ff6ece
Partial-Bug: #1280100

openstack/oslo.reports,citrix-openstack-build/oslo.reports,DirectXMan12/oslo.reports