factory: Use `dict.items` instead of `six.iteritems`

`six.iteritems` is equivalent to `dict.items` in python3, so we can
migrate all of this.

BUG=chromium:999876
TEST=make test

Change-Id: I7218c416e694da41996ae3ace6ce23e6f9fcfb72
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/2200487
Reviewed-by: Yilin Yang (kerker) <kerker@chromium.org>
Tested-by: Yilin Yang (kerker) <kerker@chromium.org>
Commit-Queue: Yilin Yang (kerker) <kerker@chromium.org>
diff --git a/py/gooftool/commands.py b/py/gooftool/commands.py
index 2f45eb2..55bae95 100755
--- a/py/gooftool/commands.py
+++ b/py/gooftool/commands.py
@@ -24,8 +24,6 @@
 import time
 import xmlrpc.client
 
-from six import iteritems
-
 from cros.factory.gooftool.common import ExecFactoryPar
 from cros.factory.gooftool.common import Shell
 from cros.factory.gooftool.core import Gooftool
@@ -901,7 +899,7 @@
   """Get firmware hash from a file"""
   if os.path.exists(options.file):
     value_dict = chromeos_firmware.CalculateFirmwareHashes(options.file)
-    for key, value in iteritems(value_dict):
+    for key, value in value_dict.items():
       print('  %s: %s' % (key, value))
   else:
     raise Error('File does not exist: %s' % options.file)