[IMP] sale_triple_discount: Compute amount with decimal precision of 16

Equivalent to the current computation in invoices
https://github.com/OCA/account-invoicing/blob/d2e12c03b50011bb788cf482346710f0fefe0de3/account_invoice_triple_discount/models/account_move_line.py#L73-L74

sale_triple_discount/models/sale_order_line.py

@@ -97,6 +97,9 @@ def triple_discount_preprocess(self):
         this method is called multiple times."""
 
         prev_values = dict()
+        digits = self._fields["discount"]._digits
+        self.env["sale.order.line"]._fields["discount"]._digits = (16, 16)
+        prev_values["discount_digits"] = digits
         for line in self:
             prev_values[line] = dict(
                 discount=line.discount,
@@ -115,6 +118,8 @@ def triple_discount_preprocess(self):
     @api.model
     def triple_discount_postprocess(self, prev_values):
         """Restore the discounts of the lines in the dictionary prev_values."""
+        digits = prev_values.pop("discount_digits")
+        self.env["sale.order.line"]._fields["discount"]._digits = digits
         for line, prev_vals_dict in list(prev_values.items()):
             line.update(prev_vals_dict)