Add fake `basestring` for Python 3 tests
Add fake `basestring` for Python 3 tests

Python 3 has no type called `basestring`. (It also doesn't have `unicode`.) While the check in `WhenTest.test_formats()` is correct for Python 2.x, `str` is needed for Python 3.

This feels a bit hackish, but it works.

dirn/When.py