python3: Add py33 to tox.ini
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 <753a8074f04ffd62a8fff7516e3ae689be8e35ad@canonical.com>

n0ano/gantt,n0ano/gantt