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

docs_src/app_testing/tutorial001_py310.py

@@ -0,0 +1,18 @@
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+app = FastAPI()
+
+
+@app.get("/")
+async def read_main():
+    return {"msg": "Hello World"}
+
+
+client = TestClient(app)
+
+
+def test_read_main():
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.json() == {"msg": "Hello World"}