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/choices.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from django_countries import Countries
 
 CHOICE_FIELD_MAX_LENGTH = 25