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/outgoing_webhook.py

@@ -4,7 +4,7 @@ import logging
 from typing import Any, AnyStr, Dict, Optional
 
 import requests
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 from requests import Response, Session
 
 from version import ZULIP_VERSION