Fix broken import

apparently when running run_test.py in my directory

   from exceptions

somehow works but not when run in CI.

Author: rouilj

roundup/mailgw.py

@@ -118,7 +118,7 @@ class node. Any parts of other types are each stored in separate files
 except ImportError:
     gpg = None
 
-from exceptions import RoundupException
+from roundup.exceptions import RoundupException
 
 SENDMAILDEBUG = os.environ.get('SENDMAILDEBUG', '')