remove legacy argument

tests/test_postfix.py

@@ -13,7 +13,7 @@ if not os.path.exists('/etc/postfix'):
     sys.exit(1)
 else:
     # check that postfix listens the port 25 correctly
-    status, out = subprocess.getstatusoutput('netstat -pant | grep master | grep 127.0.0.1:25', shell=True)
+    status, out = subprocess.getstatusoutput('netstat -pant | grep master | grep 127.0.0.1:25')
     if status != 0:
         print('The port 25 is not listened by any process.')
         sys.exit(1)