[IMP] purchase: Take into account purchase order date for sequence

**Steps to reproduce**

* Define "Purchase Order" sequence for using date ranges checking "Use subsequences per date_range".
* Change sequence prefix to "PO/%(year)s/".
* Create a purchase order with order date in 2020 (a different year than current one).

**Current behavior**

Got a purchase order with 2019 (current year) number.

**Expected behavior**

Got a purchase order with 2020 number.

closes odoo/odoo#37762

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>