code: def fresh(req_headers: HEADER_TYPE, res_headers: HEADER_TYPE) -> bool:
      
      modified_since = cast(
          Optional[str],
          req_headers.get('if-modified-since'),
      )
      none_match = cast(
          Optional[str],
          req_headers.get('if-none-match'),
      )
      if not modified_since and not none_match:
          return False
      cache_control = req_headers.get('cache-control')
      if cache_control is not None:
          if isinstance(cache_control, list):
              for control in cache_control:
                  if control.strip() == "no-cache":
                      return False
          elif cache_control.strip() == "no-cache":
              return False
      if none_match is not None and none_match != "*":
          etag = cast(
              Optional[str],
              res_headers.get('ETag') or res_headers.get('etag'),
          )
          if etag is None:
              return False
          if etag.startswith("W/"):
              etag = etag[2:]
          if none_match.startswith("W/"):
              none_match = none_match[2:]
          etag_stale = True
          matches = none_match.split(",")
          for match in matches:
              match = match.strip()
              if match == etag:
                  etag_stale = False
                  break
          if etag_stale:
              return False
      if modified_since:
          last_modified = cast(
              Optional[str],
              res_headers.get('Last-Modified') or res_headers.get('last-modified'),
          )
          modified_stale = True
          if last_modified is not None:
              last_date = parse_http_date(last_modified)
              modified_date = parse_http_date(modified_since)
              if last_date != 0 and modified_date != 0 and last_date <= modified_date:
                  modified_stale = False
          if modified_stale:
              return False
      return True
docs: 根据 req_headers, res_headers 判断改消息是否为 304