tools: remove the dependency on the future package

examples/protocols/mqtt/ssl/mqtt_ssl_example_test.py

@@ -1,10 +1,7 @@
-from __future__ import print_function, unicode_literals
-
 import os
 import re
 import ssl
 import sys
-from builtins import str
 from threading import Event, Thread
 
 import paho.mqtt.client as mqtt