[FIX] base_vat: fix constrains

The vat checking constrains uses both the country_id and the vat, but
was defined as depending only on the vat field

addons/base_vat/models/res_partner.py

@@ -150,7 +150,7 @@ class ResPartner(models.Model):
                 vat = country_code + vat
         return vat
 
-    @api.constrains('vat')
+    @api.constrains('vat', 'country_id')
     def check_vat(self):
         if self.env.context.get('company_id'):
             company = self.env['res.company'].browse(self.env.context['company_id'])