Bumped six to version 1.5.2

The `six` that is bundled with `2.3` is quite outdated and caused some problems which were quite a headache to figure out where they were coming from (see https://github.com/ipython/ipython/issues/4784 and https://github.com/mwaskom/seaborn/issues/82). To save others the pain I updated `six.py` to `1.5.2` (most recent).