python: Convert deprecated Django ugettext alias to gettext.

django.utils.translation.ugettext is a deprecated alias of
django.utils.translation.gettext as of Django 3.0, and will be removed
in Django 4.0.

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

zerver/lib/message.py

@@ -9,7 +9,7 @@ import orjson
 from django.db import connection
 from django.db.models import Max, Sum
 from django.utils.timezone import now as timezone_now
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 from psycopg2.sql import SQL
 from typing_extensions import TypedDict