Fixed py3 compat. (#1272)

* Fixed py3 compat.

see #1243

* Revert useless change.

modoboa/admin/management/commands/subcommands/_mx.py

@@ -82,8 +82,9 @@ class CheckMXRecords(BaseCommand):
         """Store DNSBL provider results for domain."""
         alerts = {}
         to_create = []
-        for mx in results.keys():
-            result = "" if not results[mx] else results[mx]
+        for mx, result in list(results.items()):
+            if not result:
+                result = ""
             dnsbl_result = models.DNSBLResult.objects.filter(
                 domain=domain, provider=provider, mx=mx).first()
             trigger = False