Add code style checks to tox tests

The code isn't entirely consistent in style at the moment.

Do you think we should add a code style check as a tox test environment so the code style is checked every time tox is run locally?

[pep438][] uses flake8 and py3kwarn for Python 3 compliance for example.

Thoughts on this concept?

[pep438]: https://github.com/treyhunner/pep438/blob/master/tox.ini