mypy: Enable new error explicit-override.

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

zerver/management/commands/create_realm_internal_bots.py

@@ -1,6 +1,7 @@
 from typing import Any
 
 from django.core.management.base import BaseCommand
+from typing_extensions import override
 
 from zerver.lib.onboarding import create_if_missing_realm_internal_bots
 
@@ -13,6 +14,7 @@ These are normally created when the realm is, so this should be a no-op
 except when upgrading to a version that adds a new realm internal bot.
 """
 
+    @override
     def handle(self, *args: Any, **options: Any) -> None:
         create_if_missing_realm_internal_bots()
         # create_users is idempotent -- it's a no-op when a given email