mypy: Enable new error explicit-override.

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

zerver/management/commands/create_stream.py

@@ -1,6 +1,8 @@
 from argparse import ArgumentParser
 from typing import Any
 
+from typing_extensions import override
+
 from zerver.lib.management import ZulipBaseCommand
 from zerver.lib.streams import create_stream_if_needed
 
@@ -11,10 +13,12 @@ class Command(ZulipBaseCommand):
 This should be used for TESTING only, unless you understand the limitations of
 the command."""
 
+    @override
     def add_arguments(self, parser: ArgumentParser) -> None:
         self.add_realm_args(parser, required=True, help="realm in which to create the stream")
         parser.add_argument("stream_name", metavar="<stream name>", help="name of stream to create")
 
+    @override
     def handle(self, *args: Any, **options: str) -> None:
         realm = self.get_realm(options)
         assert realm is not None  # Should be ensured by parser