Please do not import unittest

Great project!

Could you please do not import `unittest` framework in the library code?

https://github.com/xolox/python-humanfriendly/blob/1abbe85eb281fca17ae1cb769d0a23acdd2e1bd8/humanfriendly/compat.py#L68

Thank you!