update how to get days of week from DateTime

product/ERP5/mixin/periodicity.py

@@ -227,7 +227,11 @@ class PeriodicityMixin:
     """
     returns something like ['Sunday','Monday',...]
     """
-    return DateTime._days
+    try:
+      from DateTime.DateTime import _DAYS
+      return _DAYS
+    except ImportError: # BBB DateTime 2.12
+      return DateTime._days
 
   security.declareProtected(Permissions.AccessContentsInformation, 'getWeekDayItemList')
   def getWeekDayItemList(self):