[FIX] l10n_it_edi: receive bill that has a document discount

Bug:
Currently, trying to import a fatturapa XML that has a document discount
doesn't do anything.

Setup:
- install l10n_it_edi and account_accountant
- have a fatturapa XML that has a document discount

Steps to reproduce:
- switch to the Italian company ()
- make sure that the VAT number on the document matches the one on the
  company
- attempt to upload the XML bill

You should be met with an empty vendor bill page.

Cause:
The issue comes from here: https://github.com/odoo/odoo/blob/43c9820b1d3020d89b1b7ca016754e29d1fc6b58/addons/l10n_it_edi/models/account_edi_format.py#LL725C21-L729C83


in 16.0, `invoice_form` is not an instance of `Form`, but a record.

opw-3193634

closes odoo/odoo#121010

Signed-off-by: Josse Colpaert <jco@odoo.com>