Factor up py2/py3 switches

[erikrose]

We do things like this in a few places:

if sys.version_info[0] == 3:
    text_type = str
    basestring = str
else:
    text_type = unicode  # noqa

Let's depend on and use six. There's no reason to be afraid of dependencies.

Blocks #72.

[jquast]

Pull request #89 is still awaiting your merge to close this issue.