Add required trailers key for asgiref==3.6.0

`asgiref==3.6.0` introduced a new required `trailers` key to the
`asgiref.types.HTTPResponseStartEvent` type. As the django/asgiref#339
code review pointed out, this is a breaking change for type checking,
but it was added in a minor release, and not mentioned in the changelog.

django/asgiref#339
django/asgiref#362

inboard/app/main_base.py

@@ -41,6 +41,7 @@ async def app(
         "type": "http.response.start",
         "status": 200,
         "headers": [(b"content-type", b"text/plain")],
+        "trailers": False,
     }
     body_event: HTTPResponseBodyEvent = {
         "type": "http.response.body",