he value of the extension.
        :type value: :py:data:`bytes`

        :param subject: Optional X509 certificate to use as subject.
        :type subject: :py:class:`X509`

        :param issuer: Optional X509 certificate to use as issuer.
        :type issuer: :py:class:`X509`

        .. _extension: https://www.openssl.org/docs/manmaster/man5/
            x509v3_config.html#STANDARD-EXTENSIONS
        z