integrations: Remove usage of six.

zulip/integrations/zephyr/check-mirroring

@@ -9,7 +9,6 @@ import subprocess
 import hashlib
 import zephyr
 import zulip
-from six.moves import range
 
 if False:
     from typing import Any, Dict, List, Set, Tuple

zulip/integrations/zephyr/zephyr_mirror_backend.py

@@ -4,8 +4,6 @@ from typing import IO, Any, Dict, List, Text, Union, Set, Tuple, Optional, cast
 from types import FrameType
 
 import sys
-from six.moves import map
-from six.moves import range
 import json
 import re
 import time