Don't run python 3 tests by default

Remove py33 from the list of environments used when someone
runs "tox" without any arguments. The python 3.3 tests do
not pass yet, and the expected failure is confusing to new
contributors.

Change-Id: I1f1307153c4a32e59bcffbf340c31dbf3e70173c
Merge "Don't run python 3 tests by default"