mypy: Enable new error explicit-override.

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

zilencer/management/commands/calculate_first_visible_message_id.py

@@ -1,6 +1,7 @@
 from typing import Any, Iterable
 
 from django.core.management.base import CommandParser
+from typing_extensions import override
 
 from zerver.lib.management import ZulipBaseCommand
 from zerver.lib.message import maybe_update_first_visible_message_id
@@ -10,6 +11,7 @@ from zerver.models import Realm
 class Command(ZulipBaseCommand):
     help = """Calculate the value of first visible message ID and store it in cache"""
 
+    @override
     def add_arguments(self, parser: CommandParser) -> None:
         self.add_realm_args(parser)
         parser.add_argument(
@@ -20,6 +22,7 @@ class Command(ZulipBaseCommand):
             required=True,
         )
 
+    @override
     def handle(self, *args: Any, **options: Any) -> None:
         target_realm = self.get_realm(options)