mypy: Enable new error explicit-override.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/sqlalchemy_utils.py

@@ -4,6 +4,7 @@ from typing import Iterator, Optional
 import sqlalchemy
 from django.db import connection
 from sqlalchemy.engine import Connection, Engine
+from typing_extensions import override
 
 from zerver.lib.db import TimeTrackingConnection
 
@@ -11,6 +12,7 @@ from zerver.lib.db import TimeTrackingConnection
 # This is a Pool that doesn't close connections.  Therefore it can be used with
 # existing Django database connections.
 class NonClosingPool(sqlalchemy.pool.NullPool):
+    @override
     def status(self) -> str:
         return "NonClosingPool"