[T-2561] Fix too many values to unpack when call apply_retention

Encountered an issue when creating a new line on a payment order, resulting in the following error: ValueError: too many values to unpack (expected 2)

This error is triggered when the apply_retention function from the retention.tax.application model is called within the compute_advance_payment_retentions function.

base, amount = taxapp.apply_retention(
                self.partner_id,
                ret_vals.get('percent'),
                ret_vals.get('excluded_percent'),
                vals,
                self.date,
)

However, the apply_retention function returns three values:

return (
            base * sign_base,
            amount * sign_base,
            payment_order_base * sign_base,
        )

Detailed traceback

  File "/opt/odoo/custom/src/odoo/odoo/models.py", line 6507, in onchange
    record._onchange_eval(name, field_onchange[name], result)
  File "/opt/odoo/custom/src/odoo/odoo/models.py", line 6240, in _onchange_eval
    method_res = method(self)
  File "/opt/odoo/auto/addons/l10n_ar_eynes/models/account_payment_order.py", line 900, in onchange_payment_line
    amount = self.payment_order_amount_hook()
  File "/opt/odoo/auto/addons/l10n_ar_eynes/models/account_payment_order.py", line 445, in payment_order_amount_hook
    amount += self.compute_advance_payment_retentions(amount)
  File "/opt/odoo/auto/addons/l10n_ar_eynes/models/account_payment_order.py", line 513, in compute_advance_payment_retentions
    base, amount = taxapp.apply_retention(
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/odoo/custom/src/odoo/odoo/http.py", line 654, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo/custom/src/odoo/odoo/http.py", line 301, in _handle_exception
    raise exception.with_traceback(None) from new_cause
ValueError: too many values to unpack (expected 2)