Get the relative path for the given path from the current
        file by working around https://bugs.python.org/issue20012.

def _rel(self, path):
        """
        Get the relative path for the given path from the current
        file by working around https://bugs.python.org/issue20012.
        """
        return os.path.relpath(
            str(path), self._parent).replace(os.path.sep, '/')