I was writing a test that could return True for "if not pathname" (http://hg.python.org/cpython/file/58bd6a58365d/Lib/packaging/util.py#l109), and included all values I know that could return a False value, but then I realised that the only value we should check against is an empty string... I don't think any other type is applicable.

I left None as a valid type, otherwise some other test (test_command_install_dist.py) fails.