Replaced ugettext and ugettext_lazy with gettext and gettext_lazy

ugettext and ugettext_lazy were deprecated in Django 3 and will be
removed in Django 4.

dav_events/models/participant.py

@@ -4,7 +4,7 @@ import datetime
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.utils import timezone
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 from dav_base.validators import DAVNumberValidator