replace itertools.i{map,zip} with six.moves.{map,zip}

Until we can move to pure Python 3, go through the move module.

BUG=chromium:997354
TEST=`./run_tests` passes

Change-Id: Ifc270276dc58741abf4132ec85ff79dbb6108369
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1770426
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/dep_tracker.py b/scripts/dep_tracker.py
index b58b19b..168e775 100644
--- a/scripts/dep_tracker.py
+++ b/scripts/dep_tracker.py
@@ -20,12 +20,13 @@
 
 from __future__ import print_function
 
-import itertools
 import json
 import multiprocessing
 import os
 import stat
 
+from six.moves import map as imap
+
 from chromite.lib import commandline
 from chromite.lib import cros_logging as logging
 from chromite.lib import filetype
@@ -85,7 +86,7 @@
       self._pool = multiprocessing.Pool(jobs)
       self._imap = self._pool.map
     else:
-      self._imap = itertools.imap
+      self._imap = imap
 
     self._files = {}
     self._ebuilds = {}