[FIX] purchase: missing data in tax computation

- Create a purchase tax computed with Python code.
- The Python code must use an attribute of the product, e.g.
  `product.price`.
- Create a PO using the created purchase tax.

It crashes, because the product (as well as the partner) is not sent to
the `compute_all` method.

opw-770078

addons/purchase/models/purchase.py

@@ -649,7 +649,9 @@ class PurchaseOrderLine(models.Model):
         order = line.order_id
         price_unit = line.price_unit
         if line.taxes_id:
-            price_unit = line.taxes_id.with_context(round=False).compute_all(price_unit, currency=line.order_id.currency_id, quantity=1.0)['total_excluded']
+            price_unit = line.taxes_id.with_context(round=False).compute_all(
+                price_unit, currency=line.order_id.currency_id, quantity=1.0, product=line.product_id, partner=line.order_id.partner_id
+            )['total_excluded']
         if line.product_uom.id != line.product_id.uom_id.id:
             price_unit *= line.product_uom.factor / line.product_id.uom_id.factor
         if order.currency_id != order.company_id.currency_id: