[FIX] l10n_it_edi: self-invoices require fiscal regime RF18

When the seller is foreign, the Fiscal Regime of the seller must
be set to 'RF18', as per:
https://www.fattura24.com/manuale/altri-documenti/autofattura-fornitori/

Task: https://www.odoo.com/web#id=3010849&model=project.task


opw-3010849

closes odoo/odoo#104780

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

addons/l10n_it_edi/models/account_invoice.py

@@ -279,7 +279,7 @@ class AccountMove(models.Model):
             'document_total': document_total,
             'representative': company.l10n_it_tax_representative_partner_id,
             'codice_destinatario': codice_destinatario,
-            'regime_fiscale': company.l10n_it_tax_system if not is_self_invoice else 'RF01',
+            'regime_fiscale': company.l10n_it_tax_system if not is_self_invoice else 'RF18',
             'is_self_invoice': is_self_invoice,
             'partner_bank': self.partner_bank_id,
             'format_date': format_date,