Use six library
In my opinion, the `six` library is silly and I don't want to add a useless dependency.

You should use the wonderful [`six`](https://pythonhosted.org/six/) library, which would make a lot of [this](https://github.com/coleifer/peewee/blob/master/peewee.py#L118) unnecessary (especially the [`exec` for `reraise`](https://github.com/coleifer/peewee/blob/master/peewee.py#L143)).
