Add __table_handler__ to ModelSQL

issue7581
review48531002

payment.py

@@ -7,7 +7,6 @@ from trytond.pyson import Eval, If
 from trytond.transaction import Transaction
 from trytond.wizard import Wizard, StateView, StateAction, Button
 from trytond.pool import Pool
-from trytond import backend
 
 __all__ = ['Journal', 'Group', 'Payment',
     'ProcessPaymentStart', 'ProcessPayment']
@@ -62,8 +61,7 @@ class Group(ModelSQL, ModelView):
 
     @classmethod
     def __register__(cls, module_name):
-        TableHandler = backend.get('TableHandler')
-        table_h = TableHandler(cls, module_name)
+        table_h = cls.__table_handler__(module_name)
 
         # Migration from 3.8: rename reference into number
         if table_h.column_exist('reference'):