python3: Add py33 to tox.ini

Introduce py33 to tox.ini to make testing with
python3 easier.

Change-Id: I3f966efc1a845aaa8e317d810bd6a1366acd323c
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: Add py33 to tox.ini"