FIX: Get rid of basestring (not python3 compatible).

dav_events/forms/events.py

@@ -9,6 +9,7 @@ from django.apps import apps
 from django.utils.translation import get_language, ugettext, ugettext_lazy as _
 from django_countries.fields import Country, LazyTypedChoiceField
 from datetimewidget.widgets import DateWidget, TimeWidget, DateTimeWidget
+from six import string_types
 
 from .. import choices
 from .. import config
@@ -162,7 +163,7 @@ class EventCreateForm(ModelMixin, ChainedForm):
         kwargs = self._session_data.copy()
         if 'deadline' in kwargs:
             buf = kwargs['deadline']
-            if isinstance(buf, basestring):
+            if isinstance(buf, string_types):
                 deadline_choice = buf.lower()
                 deadline_field = 'deadline_{}'.format(deadline_choice)
                 if deadline_field in kwargs: