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/tools/ovl.py b/py/tools/ovl.py
index b1603a1..a6b4cfe 100755
--- a/py/tools/ovl.py
+++ b/py/tools/ovl.py
@@ -39,7 +39,6 @@
 import jsonrpclib
 from jsonrpclib.config import Config
 from jsonrpclib.SimpleJSONRPCServer import SimpleJSONRPCServer
-from six import iteritems
 from ws4py.client import WebSocketBaseClient
 import yaml
 
@@ -674,7 +673,7 @@
   This decorator retrieve command info from each method and append it in to the
   SUBCOMMANDS class variable, which is later used to construct parser.
   """
-  for unused_key, method in iteritems(cls.__dict__):
+  for unused_key, method in cls.__dict__.items():
     if hasattr(method, '__arg_attr'):
       # pylint: disable=protected-access
       cls.SUBCOMMANDS.append(method.__arg_attr)