✨ 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/tutorial002_py310.py

@@ -0,0 +1,10 @@
+from fastapi import FastAPI, Header
+
+app = FastAPI()
+
+
+@app.get("/items/")
+async def read_items(
+    strange_header: str | None = Header(None, convert_underscores=False)
+):
+    return {"strange_header": strange_header}