Build a handshake response to send to the client.

    ``key`` comes from :func:`check_request`.

def build_response(headers: Headers, key: str) -> None:
    """
    Build a handshake response to send to the client.

    ``key`` comes from :func:`check_request`.

    """
    headers["Upgrade"] = "websocket"
    headers["Connection"] = "Upgrade"
    headers["Sec-WebSocket-Accept"] = accept(key)