Return a new path with the file suffix changed (or added, if
        none).

def with_suffix(self, suffix):
        """Return a new path with the file suffix changed (or added, if
        none).
        """
        # XXX if suffix is None, should the current suffix be removed?
        f = self._flavour
        if f.sep in suffix or f.altsep and f.altsep in suffix:
            raise ValueError("Invalid suffix %r" % (suffix))
        if suffix and not suffix.startswith('.') or suffix == '.':
            raise ValueError("Invalid suffix %r" % (suffix))
        name = self.name
        if not name:
            raise ValueError("%r has an empty name" % (self,))
        old_suffix = self.suffix
        if not old_suffix:
            name = name + suffix
        else:
            name = name[:-len(old_suffix)] + suffix
        return self._from_parsed_parts(self._drv, self._root,
                                       self._parts[:-1] + [name])