python3: use six.text_types for unicode()

Type for representing (Unicode) textual data. This is unicode() in Python 2 and str in Python 3.

Change-Id: I8319287f3294ebf7c1a506cd4f68509d9554a5ef
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: use six.text_types for unicode()"