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

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

docs_src/body_nested_models/tutorial003_py39.py

@@ -0,0 +1,20 @@
+from typing import Optional
+
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class Item(BaseModel):
+    name: str
+    description: Optional[str] = None
+    price: float
+    tax: Optional[float] = None
+    tags: set[str] = set()
+
+
+@app.put("/items/{item_id}")
+async def update_item(item_id: int, item: Item):
+    results = {"item_id": item_id, "item": item}
+    return results