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/views/message_flags.py

@@ -1,7 +1,7 @@
 from typing import List, Optional
 
 from django.http import HttpRequest, HttpResponse
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 
 from zerver.decorator import REQ, has_request_variables
 from zerver.lib.actions import (