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/zcommand.py

@@ -1,6 +1,6 @@
 from typing import Any, Dict
 
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 
 from zerver.lib.actions import do_set_user_display_setting
 from zerver.lib.exceptions import JsonableError