[FIX] base_vat: accept XI VAT format

Following Brexit on January 1st 2021, companies in Northern Ireland have
a new VAT number starting with XI instead of GB. More info:
https://www.gov.uk/government/publications/accounting-for-vat-on-goods-moving-between-great-britain-and-northern-ireland-from-1-january-2021/check-when-you-are-trading-under-the-northern-ireland-protocol-if-you-are-vat-registered-business

stdnum support the new XI VAT from 1.16
arthurdejong/python-stdnum@b93d695
This patch add temporary support in base_vat until the new version
is available on the Debian package repository

Community tracked issue
#64891

opw-2461322

closes #67064

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

addons/base_vat/models/res_partner.py

@@ -47,7 +47,7 @@
     'es': 'ESA12345674',
     'fi': 'FI12345671',
     'fr': 'FR32123456789',
-    'gb': 'GB123456782',
+    'gb': 'GB123456782 or XI123456782',
     'gr': 'GR12345670',
     'hu': 'HU12345676',
     'hr': 'HR01234567896',  # Croatia, contributed by Milan Tribuson
@@ -430,3 +430,10 @@ def check_vat_ua(self, vat):
                 else:
                     res.append(False)
         return all(res)
+
+    def check_vat_xi(self, vat):
+        """ Temporary Nothern Ireland VAT validation following Brexit
+        As of January 1st 2021, companies in Northern Ireland have a
+        new VAT number starting with XI
+        TODO: remove when stdnum is updated to 1.16 in supported distro"""
+        return stdnum.util.get_cc_module('gb', 'vat').is_valid(vat) if stdnum else True