bundles/postfix: use dig in check_spam_blocklist instead of a python library

The library isn't available as a debian package, so we would have to
manually install that every time the python package updates its minor
version number.

bundles/postfix/files/check_spam_blocklist

@@ -2,10 +2,9 @@
 
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from ipaddress import ip_address, IPv6Address
+from subprocess import check_output
 from sys import argv, exit
 
-from dns.exception import Timeout
-from dns.resolver import Resolver, NoAnswer, NXDOMAIN, NoNameservers
 
 
 BLOCKLISTS = [
@@ -33,10 +32,6 @@ BLOCKLISTS = [
 ]
 
 def check_list(ip_list, blocklist):
-    resolver = Resolver()
-    resolver.timeout = 5
-    resolver.lifetime = 5
-
     dns_name = '{}.{}'.format(
         '.'.join(ip_list),
         blocklist,
@@ -46,7 +41,13 @@ def check_list(ip_list, blocklist):
     msgs = []
 
     try:
-        result = resolver.query(dns_name)
+        result = check_output([
+            'dig',
+            '+tries=2',
+            '+time=5',
+            '+short',
+            dns_name
+        ]).decode().splitlines()
         for item in result:
             msgs.append('{} listed in {} as {}'.format(
                 ip,
@@ -54,10 +55,11 @@ def check_list(ip_list, blocklist):
                 item,
             ))
             returncode = 2
-    except (NoAnswer, NXDOMAIN, NoNameservers, Timeout):
-        # Probably fine
-        pass
     except Exception as e:
+        if e.returncode == 9:
+            # no reply from server
+            pass
+
         return [repr(e)], 3
 
     return msgs, returncode