def add_extension(self, extension, critical):
        """
        Adds an X.509 extension to the certificate.
        """
        if not isinstance(extension, ExtensionType):
            raise TypeError("extension must be an ExtensionType")

        extension = Extension(extension.oid, critical, extension)
        _reject_duplicate_extension(extension, self._extensions)

        return CertificateBuilder(
            self._issuer_name, self._subject_name,
            self._public_key, self._serial_number, self._not_valid_before,
            self._not_valid_after, self._extensions + [extension]
        )

        Adds an X.509 extension to the certificate.
        
stackoverflow/venv/lib/python3.6/site-packages/cryptography/x509/base.py