code: def fix_timezone_separator(cls, timestr):
          
          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
docs: Replace invalid timezone separator to prevent
          `dateutil.parser.parse` to raise.
  
          :return: the new string if invalid separators were found,
                   `None` otherwise