docs: def retrieve(url):
      """Retrieve and parse PEM-encoded X.509 certificate chain.
  
      See `validate.request` for additional info.
  
      Args:
          url: str. SignatureCertChainUrl header value sent by request.
  
      Returns:
          list or bool: If url is valid, returns the certificate chain as a list
              of cryptography.hazmat.backends.openssl.x509._Certificate
              certificates where certs[0] is the first certificate in the file; if
              url is invalid, returns False.
      """
      try:
          pem_data = urlopen(url).read()
      except (ValueError, HTTPError):
          warnings.warn('Certificate URL is invalid.')
          return False
  
      if sys.version >= '3':
          try:
              pem_data = pem_data.decode()
          except(UnicodeDecodeError):
              warnings.warn('Certificate encoding is not utf-8.')
              return False
  
      return _parse_pem_data(pem_data)