chore: checkpoint before Python removal

vendor/tower/tests/util/service_fn.rs

@@ -0,0 +1,12 @@
+use std::future::ready;
+use tower::util::service_fn;
+use tower_service::Service;
+
+#[tokio::test(flavor = "current_thread")]
+async fn simple() {
+    let _t = super::support::trace_init();
+
+    let mut add_one = service_fn(|req| ready(Ok::<_, ()>(req + 1)));
+    let answer = add_one.call(1).await.unwrap();
+    assert_eq!(answer, 2);
+}