[FIX] account_edi_ubl_cii: invoice lines could have multiple fixed taxes

An UBL Bis 3 xml can only contain one tax per invoice line. However,
there might be one or more ecotaxes on invoice lines in Belgium. Do not
block the user from generating the attachment in this case.

Note that PR https://github.com/odoo/odoo/pull/121494

 will properly
handle the fixed taxes upon generating UBL attachments.

opw-3318969

closes odoo/odoo#121833

Signed-off-by: William André (wan) <wan@odoo.com>

addons/account_edi_ubl_cii/models/account_edi_xml_ubl_bis3.py

@@ -351,9 +351,10 @@ class AccountEdiXmlUBLBIS3(models.AbstractModel):
             ) if intracom_delivery else None,
         }
 
-        for line in invoice.line_ids:
-            if len(line.tax_ids) > 1:
+        for line in invoice.invoice_line_ids:
+            if len(line.tax_ids.filtered(lambda t: t.amount_type != 'fixed')) != 1:
                 # [UBL-SR-48]-Invoice lines shall have one and only one classified tax category.
+                # /!\ exception: possible to have any number of ecotaxes (fixed tax) with a regular percentage tax
                 constraints.update({'cen_en16931_tax_line': _("Each invoice line shall have one and only one tax.")})
 
         return constraints