Fix Python 3.3 jobs on Travis.

[GabrielC101]

As @benoit-pierre mentioned in pypa/pip#4897, the most current version of pytest no longer supports Python 3.3. This causes Travis to fail.

This PR applies a fix similar to the one used in pypa/pip#4897.

[GabrielC101]

Could someone review/approve this? It's urgent, but uncontroversial.

[hugovk]

Pytest dropped a Python 3.3 as it's EOL, no longer receiving security updates, and is generally little used (see pypa/pip#3796 for 1+ year old numbers).

Does this library still need to support 3.3? How about just dropping it, or at the very least deprecating it?

[GabrielC101]

Pulling support for python3.3 is an option . . . but another approach is to stop testing for python3.3 in tox /travis, but allow python3.3 in a "use at your own risk capacity."

In other words, change tox.ini and .travis.yml, but leave setup.py alone.

In fact, now that I look at it, the only reference to Python version in setup.py is in the classifiers.

So what does "supporting" python3.3 mean?

[hugovk]

This can be closed now #118 is merged.