[FIX] account: Wrong tax amount with Python tax code

Steps to reproduce the bug:

- Create a new tax T based on python code, leave the code how it is by default (result = price_unit * 0.10)
- Check included in price
- Create an invoice, add a customer, add a line with random label, assign unit price = 100 and select tax T.

Bug:

On the invoice, the Untaxed Amount =  90 and Taxes = -10 instead of 10 (like the behavior in 12.0)

opw:2242188

closes odoo/odoo#50485

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>