It looks like Python 3.x's relpath isn't compatible between posixpath
and ntpath. In posixpath.relpath, the start keyword defaults to None,
but ntpath.relpath, the start keyword defaults to curdir.

Interestingly enough, 2.6 and 2.7 have a different implementation of
posixpath.relpath, where it explicitly sets the start to equal curdir,
just like ntpath does.