Always replace `unicode()` with `six.text_type`

This would go against the current practice in the other fixers of only doing `from six import ...` statements when doing global overrides, e.g. for `map`. Plus not using 'six' is such a pain even I gave up trying after all those issues I filed about it.

But if your leanings are more requirements I'll change the code (I obviously have a personal aversion to importing things directly from modules which is why I'm pushing back a little =).