test_smtp inconsistency for py3+IPv6

It appears that, under Python3, on an IPv6 enabled machine,
the testing SMTP server on 'localhost' can turn out to listen on ::1 only,
which makes those tests break if the SMTP client part uses 127.0.0.1
directly. Using 'localhost' there as well makes the tests pass.

fail2ban/tests/action_d/test_smtp.py

@@ -67,7 +67,7 @@ class SMTPActionTest(unittest.TestCase):
 		port = self.smtpd.socket.getsockname()[1]
 
 		self.action = customActionModule.Action(
-			self.jail, "test", host="127.0.0.1:%i" % port)
+			self.jail, "test", host="localhost:%i" % port)
 
 		## because of bug in loop (see loop in asyncserver.py) use it's loop instead of asyncore.loop:
 		self._active = True