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/domains.py

@@ -2,7 +2,7 @@ import re
 from typing import Optional
 
 from django.core.exceptions import ValidationError
-from django.utils.translation import ugettext as _
+from django.utils.translation import gettext as _
 
 
 def validate_domain(domain: Optional[str]) -> None: