python3: Introduce py33 to tox.ini

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

Change-Id: I11801a5b3fd736d8e647a8a7152e498f505108f6
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: Introduce py33 to tox.ini"