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

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

docs_src/body/tutorial003_py310.py

@@ -0,0 +1,17 @@
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+
+class Item(BaseModel):
+    name: str
+    description: str | None = None
+    price: float
+    tax: float | None = None
+
+
+app = FastAPI()
+
+
+@app.put("/items/{item_id}")
+async def create_item(item_id: int, item: Item):
+    return {"item_id": item_id, **item.dict()}