✨ Add docs and tests for Python 3.9 and Python 3.10 (#3712)

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

docs_src/background_tasks/tutorial002_py310.py

@@ -0,0 +1,24 @@
+from fastapi import BackgroundTasks, Depends, FastAPI
+
+app = FastAPI()
+
+
+def write_log(message: str):
+    with open("log.txt", mode="a") as log:
+        log.write(message)
+
+
+def get_query(background_tasks: BackgroundTasks, q: str | None = None):
+    if q:
+        message = f"found query: {q}\n"
+        background_tasks.add_task(write_log, message)
+    return q
+
+
+@app.post("/send-notification/{email}")
+async def send_notification(
+    email: str, background_tasks: BackgroundTasks, q: str = Depends(get_query)
+):
+    message = f"message to {email}\n"
+    background_tasks.add_task(write_log, message)
+    return {"message": "Message sent"}