Fixed validator classes to make them supported in migrations

src/pretix/base/validators.py

@@ -1,4 +1,5 @@
 from django.core.exceptions import ValidationError
+from django.utils.deconstruct import deconstructible
 from django.utils.translation import ugettext_lazy as _
 
 
@@ -16,6 +17,7 @@ class BlacklistValidator:
             )
 
 
+@deconstructible
 class EventSlugBlacklistValidator(BlacklistValidator):
 
     blacklist = [
@@ -31,6 +33,7 @@ class EventSlugBlacklistValidator(BlacklistValidator):
     ]
 
 
+@deconstructible
 class OrganizerSlugBlacklistValidator(BlacklistValidator):
 
     blacklist = [