def _create_ssl_context(self) -> Optional[SSLContext]:
        """
        创建 ssl
        """
        ssl_context = None
        if self.cfg.is_ssl:
            ssl_context = SSLContext(self.cfg.ssl_version)
            ssl_context.load_cert_chain(self.cfg.certfile, self.cfg.keyfile)
            if self.cfg.ca_certs:
                ssl_context.load_verify_locations(self.cfg.ca_certs)
            if self.cfg.ciphers:
                ssl_context.set_ciphers(self.cfg.ciphers)
            ssl_context.set_alpn_protocols(['http/1.1'])
        return ssl_context