Python3: Use six.StringIO for io.Bytes()

The newer version of six (1.4.1) provides six.StringIO which
is a fake file object for textual data. It's an alias for StringIO.StringIO
in python2 and io.StringIO in Python3.

Use the fake object where approiate.

Change-Id: I364001933b4f2305ac27b293a9a4a3fec36c8a49
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "Python3: Use six.StringIO for io.Bytes()"