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/event.py

@@ -13,7 +13,7 @@ from django.contrib.auth import get_user_model
 from django.urls import reverse
 from django.db import models
 from django.template.loader import get_template
-from django.utils.translation import get_language, ugettext_lazy as _
+from django.utils.translation import get_language, gettext_lazy as _
 from django_countries.fields import Country, CountryField
 
 from .. import choices