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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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