[FEAT] Travis: Check Python 3

With b9d252ec558531816f18d05adfce9e9559af7421 Python 3 support is
possible and only the pwb_tests fail. This runs now also a 3.3 test
on Travis but skips pwb_tests on Python 3.

Change-Id: If7fd171c64f612c780bee00beb46d20fd45d1fae