Predicate StdlibPrivate::WsgirefSimpleServer::writeFunction

Gets a reference to the write function (that will write data to the response), which is the return value from calling the start_response function.

See https://github.com/python/cpython/blob/b567b9d74bd9e476a3027335873bb0508d6e450f/Lib/wsgiref/handlers.py#L225-L252