Tell Travis to test in Python 3
Tell Travis to test in Python 3

[Flask tests in 3.3 through 3.6][0], so let's do the same.

[0]: https://github.com/pallets/flask/blob/7c510199850957161240f081359e2246d8982be1/.travis.yml

eyeseast/tarbell,tarbell-project/tarbell,tarbell-project/tarbell,eyeseast/tarbell