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

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

docs_src/header_params/tutorial003_py39.py

@@ -0,0 +1,10 @@
+from typing import Optional
+
+from fastapi import FastAPI, Header
+
+app = FastAPI()
+
+
+@app.get("/items/")
+async def read_items(x_token: Optional[list[str]] = Header(None)):
+    return {"X-Token values": x_token}