[FIX] account,sale: fiscal position map tax incl. to incl.

Create a 21% tax included in price
Create a 6% tax included in price
Create a fiscal position that will replace the 21% tax included by the
6% tax included
Create a customer and assign this fiscal position
Create a product :
Price = 121€
Tax = 21% tax included
Create a SO with this customer and this product
Create an invoice (from scratch) with this customer and this product

SO : the unit price is 100,00€
Invoice : the unit price is 106,00€

Fix by creating a common method to calculate the price_unit

opw-2699793

closes odoo/odoo#86514

X-original-commit: 8ab56af6827a640a3f41a87905778432b3786c00
Signed-off-by: Laurent Smet <las@odoo.com>
Signed-off-by: Grazioso Andrea (agr) <agr@odoo.com>