factory: Migrate dict.iteritems() to py2/3 compatible semantic

`dict.iteritems()` is deprecated in python3.

There is a py2/3 compatibility utilities called six module.
And we can use the `iteritems()` in this module.

BUG=chromium:999876
TEST=make test
TEST=make lint with enable=dict-iter-method on
TEST=umpire test

Change-Id: I5612d24d063f23f7f1d8e6cd39c3e1776605f69c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/1826580
Reviewed-by: Cheng-Han Yang <chenghan@chromium.org>
Tested-by: Yilin Yang <kerker@chromium.org>
Auto-Submit: Yilin Yang <kerker@chromium.org>
Commit-Queue: Yilin Yang <kerker@chromium.org>
diff --git a/py/gooftool/commands.py b/py/gooftool/commands.py
index 6d47682..2325eaa 100755
--- a/py/gooftool/commands.py
+++ b/py/gooftool/commands.py
@@ -24,6 +24,8 @@
 import time
 import xmlrpclib
 
+from six import iteritems
+
 import factory_common  # pylint: disable=unused-import
 from cros.factory.gooftool.common import ExecFactoryPar
 from cros.factory.gooftool.common import Shell
@@ -853,7 +855,7 @@
   """Get firmware hash from a file"""
   if os.path.exists(options.file):
     value_dict = chromeos_firmware.CalculateFirmwareHashes(options.file)
-    for key, value in value_dict.iteritems():
+    for key, value in iteritems(value_dict):
       print('  %s: %s' % (key, value))
   else:
     raise Error('File does not exist: %s' % options.file)