code: def last_modified(self) -> Optional[datetime.datetime]:
          
          httpdate = self._headers.get(hdrs.LAST_MODIFIED)
          if httpdate is not None:
              timetuple = parsedate(httpdate)
              if timetuple is not None:
                  return datetime.datetime(*timetuple[:6],
                                           tzinfo=datetime.timezone.utc)
          return None
docs: The value of Last-Modified HTTP header, or None.
  
          This header is represented as a `datetime` object.