📝 Update source examples and docs from Python 3.9 to 3.10 (#14900)

docs_src/app_testing/tutorial003_py310.py

@@ -0,0 +1,24 @@
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+app = FastAPI()
+
+items = {}
+
+
+@app.on_event("startup")
+async def startup_event():
+    items["foo"] = {"name": "Fighters"}
+    items["bar"] = {"name": "Tenders"}
+
+
+@app.get("/items/{item_id}")
+async def read_items(item_id: str):
+    return items[item_id]
+
+
+def test_read_items():
+    with TestClient(app) as client:
+        response = client.get("/items/foo")
+        assert response.status_code == 200
+        assert response.json() == {"name": "Fighters"}