Replace deprecated @app.on_event("startup") with lifespan context manager

Refactor FastAPI app initialization to use async lifespan.

docs_src/sql_databases/tutorial002_an_py310.py

@@ -1,5 +1,5 @@
+from contextlib import asynccontextmanager
 from typing import Annotated
-
 from fastapi import Depends, FastAPI, HTTPException, Query
 from sqlmodel import Field, Session, SQLModel, create_engine, select
 
@@ -45,12 +45,13 @@ def get_session():
 
 
 SessionDep = Annotated[Session, Depends(get_session)]
-app = FastAPI()
-
-
-@app.on_event("startup")
-def on_startup():
+@asynccontextmanager
+async def lifespan(app: FastAPI):
     create_db_and_tables()
+    yield
+    
+
+app = FastAPI(lifespan=lifespan)
 
 
 @app.post("/heroes/", response_model=HeroPublic)