Replace invalid timezone separator to prevent
        `dateutil.parser.parse` to raise.

        :return: the new string if invalid separators were found,
                 `None` otherwise

def fix_timezone_separator(cls, timestr):
        """Replace invalid timezone separator to prevent
        `dateutil.parser.parse` to raise.

        :return: the new string if invalid separators were found,
                 `None` otherwise
        """
        tz_sep = cls.TIMEZONE_SEPARATOR.match(timestr)
        if tz_sep is not None:
            return tz_sep.group(1) + tz_sep.group(2) + ':' + tz_sep.group(3)
        return timestr