fix using deprecated Starlette feature

`on_startup`/`on_shutdown` will be removed in v1.0.

unwind/web.py

@@ -1,4 +1,5 @@
 import asyncio
+import contextlib
 import logging
 import secrets
 from json.decoder import JSONDecodeError
@@ -612,6 +613,13 @@ def auth_error(request, err):
     return unauthorized(str(err))
 
 
+@contextlib.asynccontextmanager
+async def lifespan(app: Starlette):
+    await open_connection_pool()
+    yield
+    await close_connection_pool()
+
+
 def create_app():
     if config.loglevel == "DEBUG":
         logging.basicConfig(
@@ -622,8 +630,7 @@ def create_app():
         log.debug(f"Log level: {config.loglevel}")
 
     return Starlette(
-        on_startup=[open_connection_pool],
-        on_shutdown=[close_connection_pool],
+        lifespan=lifespan,
         routes=[
             Mount(f"{config.api_base}v1", routes=route.registered),
         ],