[16.0] Inventory Users cannot Validate receipt pickings [purchase_discount]

[atumer]

Users that only have Stock access rights, with no Purchase access rights, cannot validate Receipt pickings, due to calculation in price_unit.

Problem is related to purchase_discount module's hack in price unit calculation method for stock.move. This hack tries to write on purchase order lines temporarily, but standard inventory user does not have any "write" permission on Purchase Order Line. It is also not viable to set these users as Purchase users, just to bypass this problem.

    def _get_price_unit(self):
        """Get correct price with discount replacing current price_unit
        value before calling super and restoring it later for assuring
        maximum inheritability.

        HACK: This is needed while https://github.com/odoo/odoo/pull/29983
        is not merged.
        """
        price_unit = False
        po_line = self.purchase_line_id
        if po_line and self.product_id == po_line.product_id:
            price = po_line._get_discounted_price_unit()
            if price != po_line.price_unit:
                # Only change value if it's different
                price_unit = po_line.price_unit
                po_line.price_unit = price  # LINE CAUSING PROBLEM
        res = super()._get_price_unit()
        if price_unit:
            po_line.price_unit = price_unit  # LINE CAUSING PROBLEM
        return res