Use the legal notice of tax as tax exemption reason in UBL invoice

Closes #14572

modules/edocument_ubl/edocument.py

@@ -190,14 +190,16 @@ class Invoice(Model):
     def taxes(self):
         class Tax(namedtuple(
                     'Tax',
-                    ('type', 'rate', 'unece_category_code', 'unece_code'))):
+                    ('type', 'rate', 'unece_category_code', 'unece_code',
+                        'legal_notice'))):
             @classmethod
             def from_line(cls, line):
                 return Tax(
                     type=line.tax.type,
                     rate=line.tax.rate,
                     unece_category_code=line.tax.unece_category_code,
-                    unece_code=line.tax.unece_code)
+                    unece_code=line.tax.unece_code,
+                    legal_notice=line.legal_notice or '')
         TaxLine = namedtuple('TaxLine', ('base', 'amount', 'tax'))
         for group, lines in groupby(sorted(
                     self.invoice.taxes,

modules/edocument_ubl/template/2/base.xml

@@ -84,6 +84,7 @@ this repository contains the full copyright notices and license terms. -->
     <py:def function="TaxCategory(tax)">
         <cbc:ID py:if="tax.unece_category_code">${tax.unece_category_code}</cbc:ID>
         <cbc:Percent py:if="tax.type == 'percentage'">${format((tax.rate * 100).normalize(), 'f')}</cbc:Percent>
+        <cbc:TaxExemptionReason py:if="tax.legal_notice">${tax.legal_notice}</cbc:TaxExemptionReason>
         <cac:TaxScheme py:if="tax.unece_code">
             <cbc:ID>${tax.unece_code}</cbc:ID>
         </cac:TaxScheme>