update how to get days of week from DateTime

product/ERP5/mixin/periodicity.py

@@ -227,6 +227,10 @@ class PeriodicityMixin:
     """
     returns something like ['Sunday','Monday',...]
     """
+    try:
+      from DateTime.DateTime import _DAYS
+      return _DAYS
+    except ImportError: # BBB DateTime 2.12
       return DateTime._days
 
   security.declareProtected(Permissions.AccessContentsInformation, 'getWeekDayItemList')