Updated openstack/openstack
Project: openstack/horizon  990f151cdc16af7ca81e0fc61231344c3b4d8ff2
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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: I1d4db56dd082916879b851f2d9ac71d17c0db94f
Closes-Bug: #1280100