prepare test for porting

mixin.py

@@ -23,6 +23,8 @@ DEPENDS = ['state', 'state_cashbook']
 class SecondCurrencyMixin:
     """ two fields for 2nd currency: amount + rate
     """
+    __slots__ = ()
+
     amount_2nd_currency = fields.Numeric(
         string='Amount Second Currency',
         digits=(16, Eval('currency2nd_digits', 2)),
@@ -202,6 +204,8 @@ class SecondCurrencyMixin:
 class MemCacheIndexMx:
     """ add index to 'create_date' + 'write_date'
     """
+    __slots__ = ()
+
     @classmethod
     def __setup__(cls):
         super(MemCacheIndexMx, cls).__setup__()