Do not fail on DNSException when running test

Closes #12628

trytond/setup.py

@@ -160,7 +160,7 @@ setup(name=name,
         'barcode': ['python-barcode[images]'],
         'qrcode': ['qrcode[pil]', 'webcolors'],
         'completion': ['argcomplete'],
-        'email-validation': ['email-validator >= 2'],
+        'email-validation': ['email-validator >= 2', 'dnspython'],
         },
     zip_safe=False,
     cmdclass={

trytond/trytond/tools/email_.py

@@ -30,10 +30,17 @@ def set_from_header(message, sender, from_):
 
 
 try:
+    from dns.exception import DNSException
     from email_validator import EmailNotValidError, caching_resolver
     from email_validator import validate_email as _validate_email
 
-    resolver = caching_resolver()
+    try:
+        resolver = caching_resolver()
+    except DNSException:
+        if Pool.test:
+            resolver = None
+        else:
+            raise
 
     def validate_email(email):
         emailinfo = _validate_email(