Replace `future` with `six`

[guillochon]

This will remove a package dependency on future, which few people will have installed.

[lzkelley]

future (as in https://pypi.python.org/pypi/future) is used in places, instead of __future__ (https://docs.python.org/2/library/future.html)?

[guillochon]

Correct, it's actually imported as past, which is a part of the future package. It's mostly for string-related issues.