Fix AiohttpClientMockResponse.release (#98458)

tests/test_util/aiohttp.py

@@ -255,7 +255,7 @@ async def json(self, encoding="utf-8", content_type=None, loads=json_loads):
         """Return mock response as a json."""
         return loads(self.response.decode(encoding))
 
-    def release(self):
+    async def release(self):
         """Mock release."""
 
     def raise_for_status(self):