docs: def _offset_dt(cls, dt, offset_str):
          """
          Return a |datetime| instance that is offset from datetime *dt* by
          the timezone offset specified in *offset_str*, a string like
          ``'-07:00'``.
          """
          match = cls._offset_pattern.match(offset_str)
          if match is None:
              raise ValueError(
                  "'%s' is not a valid offset string" % offset_str
              )
          sign, hours_str, minutes_str = match.groups()
          sign_factor = -1 if sign == '+' else 1
          hours = int(hours_str) * sign_factor
          minutes = int(minutes_str) * sign_factor
          td = timedelta(hours=hours, minutes=minutes)
          return dt + td