python3: Introduce py33 to tox.ini

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

Change-Id: I96d1ecd3f0069295e27127239c83afc32673ffec
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: Introduce py33 to tox.ini"