Prepare NApp to be uploaded by creating openAPI skeleton.

def prepare(cls):
        """Prepare NApp to be uploaded by creating openAPI skeleton."""
        if cls._ask_openapi():
            napp_path = Path()
            tpl_path = SKEL_PATH / 'napp-structure/username/napp'
            OpenAPI(napp_path, tpl_path).render_template()
            print('Please, update your openapi.yml file.')
            sys.exit()