Create translations from file extension to archive format

        Returns
        -------
        dict
            The mapping from file extension to archive format
        dict
            The mapping from archive format to default file extension

def _archive_extensions():
        """Create translations from file extension to archive format

        Returns
        -------
        dict
            The mapping from file extension to archive format
        dict
            The mapping from archive format to default file extension
        """
        if six.PY3:
            ext_map = {}
            fmt_map = {}
            for key, exts, desc in shutil.get_unpack_formats():
                fmt_map[key] = exts[0]
                for ext in exts:
                    ext_map[ext] = key
        else:
            ext_map = {'.tar': 'tar',
                       '.tar.bz2': 'bztar',
                       '.tar.gz': 'gztar',
                       '.tar.xz': 'xztar',
                       '.tbz2': 'bztar',
                       '.tgz': 'gztar',
                       '.txz': 'xztar',
                       '.zip': 'zip'}
            fmt_map = {'bztar': '.tar.bz2',
                       'gztar': '.tar.gz',
                       'tar': '.tar',
                       'xztar': '.tar.xz',
                       'zip': '.zip'}
        return ext_map, fmt_map