Python 3 Fix: use bytesIO instead of cStringIO
Python 3 Fix: use bytesIO instead of cStringIO

With the move from cStringIO to the io module, use six..BytesIO as
a python 3 compatible version of cStringIO module.

Change-Id: Idc62004b28f79c5ef2e3aa0250fce30882f4be65

wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,wikimedia/integration-zuul