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

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

docs_src/extra_models/tutorial003_py310.py

@@ -0,0 +1,35 @@
+from typing import Union
+
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class BaseItem(BaseModel):
+    description: str
+    type: str
+
+
+class CarItem(BaseItem):
+    type = "car"
+
+
+class PlaneItem(BaseItem):
+    type = "plane"
+    size: int
+
+
+items = {
+    "item1": {"description": "All my friends drive a low rider", "type": "car"},
+    "item2": {
+        "description": "Music is my aeroplane, it's my aeroplane",
+        "type": "plane",
+        "size": 5,
+    },
+}
+
+
+@app.get("/items/{item_id}", response_model=Union[PlaneItem, CarItem])
+async def read_item(item_id: str):
+    return items[item_id]