Use explicit key when computing taxes and object instead of dict

This defines explicitly the available attribute and unify the use the values as
tax line instead of the keys.

modules/account_tax_non_deductible/account.py

@@ -48,7 +48,7 @@ class InvoiceLine(metaclass=PoolMeta):
                 with Transaction().set_context(
                         _non_deductible=True, _deductible_rate=1):
                     tax_amount = sum(
-                        t['amount'] for t in self._get_taxes().values())
+                        t.amount for t in self._get_taxes().values())
                 amount += tax_amount
         return amount
 
@@ -87,7 +87,7 @@ class InvoiceLine(metaclass=PoolMeta):
             taxes = self._get_taxes().values()
             for tax in taxes:
                 for type_, amount in [('base', 'base'), ('tax', 'amount')]:
-                    amount = tax[amount]
+                    amount = getattr(tax, amount)
                     with Transaction().set_context(
                             date=self.invoice.currency_date):
                         amount = Currency.compute(
@@ -96,6 +96,6 @@ class InvoiceLine(metaclass=PoolMeta):
                     tax_line = TaxLine()
                     tax_line.amount = amount
                     tax_line.type = type_
-                    tax_line.tax = tax['tax']
+                    tax_line.tax = tax.tax
                     tax_lines.append(tax_line)
         return tax_lines