Add __table_handler__ to ModelSQL

issue7581
review48531002

complaint.py

@@ -7,7 +7,6 @@ from trytond.model import ModelSQL, ModelView, Workflow, fields
 from trytond.pyson import Eval, If, Bool
 from trytond.pool import Pool
 from trytond.transaction import Transaction
-from trytond import backend
 
 from trytond.modules.product import price_digits
 
@@ -140,8 +139,7 @@ class Complaint(Workflow, 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')

sale.py

@@ -48,7 +48,7 @@ class ConfigurationSequence(metaclass=PoolMeta):
         TableHandler = backend.get('TableHandler')
         exist = TableHandler.table_exist(cls._table)
         if exist:
-            table = TableHandler(cls, module_name)
+            table = cls.__table_handler__(module_name)
             exist &= table.column_exist('complaint_sequence')
 
         super(ConfigurationSequence, cls).__register__(module_name)