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

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

docs_src/response_model/tutorial004_py39.py

@@ -0,0 +1,26 @@
+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: float = 10.5
+    tags: list[str] = []
+
+
+items = {
+    "foo": {"name": "Foo", "price": 50.2},
+    "bar": {"name": "Bar", "description": "The bartenders", "price": 62, "tax": 20.2},
+    "baz": {"name": "Baz", "description": None, "price": 50.2, "tax": 10.5, "tags": []},
+}
+
+
+@app.get("/items/{item_id}", response_model=Item, response_model_exclude_unset=True)
+async def read_item(item_id: str):
+    return items[item_id]