Remove use_2to3, use unicode_literals

This is an alternative to https://github.com/Frozenball/pytest-sugar/pull/13 for eliminating `use_2to3=True` from `setup.py`.

* `setup.py`: Remove `use_2to3=True`, remove Python 2.5
* `pytest_sugar.py`: Add `from __future__ import unicode_literals`

Python 2.5 support is removed because it doesn't support `from __future__ import unicode_literals` (see http://docs.python.org/2/library/__future__.html)

Cc: @Frozenball, @kevinburke