This looks reasonable.  Note that the output of gettempdir is always passed as first argument to os.path.join (possibly via _mkstemp_inner), so perhaps you should rather define something like

def _absolute_child_of_parent_or_tmpdir(parent, *args):
    """Return the absolute child of parent, or gettempdir() if parent is None, given by *args.
    """
    if parent is None:
        parent = <_sanitize_dir> # inline the code here
    return _os.path.join(parent, *args)

and use that function instead.

This factorizes the code a little bit more and makes intent clearer (I don't think _sanitize_dir is a very clear name).