Prevent assign unknown attributes on Model

by adding __slots__ on proteus classes.

issue6573
review254081002

tests/scenario_sale.rst

@@ -88,7 +88,6 @@ Create product::
     >>> template.name = 'product'
     >>> template.default_uom = unit
     >>> template.type = 'goods'
-    >>> template.purchasable = True
     >>> template.salable = True
     >>> template.list_price = Decimal('10')
     >>> template.account_category = account_category_tax