code: def _add_tzinfo(self, datetime_obj, tz_string):
          
          if datetime_obj is None:
              return None
  
          tzinfo_match = tz.gettz(tz_string)
          return datetime_obj.replace(tzinfo=tzinfo_match)
docs: take a naive datetime and add dateutil.tz.tzinfo object
  
          :param datetime_obj: naive datetime object
          :return: datetime object with tzinfo