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_base/validators.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 from django.core.validators import RegexValidator
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 
 DAVNumberValidator = RegexValidator(r'^'