Remove use_2to3, use unicode_literals

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)
Merge pull request #17 from Frozenball/from_future_import_unicode_literals

Remove use_2to3, use unicode_literals