✨ 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/tutorial003_py310.py

@@ -0,0 +1,22 @@
+from fastapi import FastAPI
+from pydantic import BaseModel, EmailStr
+
+app = FastAPI()
+
+
+class UserIn(BaseModel):
+    username: str
+    password: str
+    email: EmailStr
+    full_name: str | None = None
+
+
+class UserOut(BaseModel):
+    username: str
+    email: EmailStr
+    full_name: str | None = None
+
+
+@app.post("/user/", response_model=UserOut)
+async def create_user(user: UserIn):
+    return user