return an aclose-able AsyncGenerator from Template.generate_async

Fixes pallets#1955

src/jinja2/environment.py

@@ -1357,7 +1357,7 @@ async def to_list() -> t.List[str]:
 
     async def generate_async(
         self, *args: t.Any, **kwargs: t.Any
-    ) -> t.AsyncIterator[str]:
+    ) -> t.AsyncGenerator[str, object]:
         """An async version of :meth:`generate`.  Works very similarly but
         returns an async iterator instead.
         """