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/utils/schema.py b/py/utils/schema.py
index 7b8a7f9..a8d0dad 100644
--- a/py/utils/schema.py
+++ b/py/utils/schema.py
@@ -50,8 +50,6 @@
 
 import copy
 
-from six import iteritems
-
 from .type_utils import MakeList
 
 # To simplify portability issues, validating JSON schema is optional.
@@ -233,7 +231,7 @@
       raise SchemaException('Size mismatch on %r: expected size <= %r' %
                             (self.label, self.max_size))
 
-    for k, v in iteritems(data):
+    for k, v in data.items():
       self.key_type.Validate(k)
       self.value_type.Validate(v)
 
@@ -305,7 +303,7 @@
                             (self.label, type(data)))
     data_key_list = list(data)
     # Check that every key-value pair in items exists in data
-    for key, value_schema in iteritems(self.items):
+    for key, value_schema in self.items.items():
       if key not in data:
         raise SchemaException(
             'Required item %r does not exist in FixedDict %r' %
@@ -314,7 +312,7 @@
       data_key_list.remove(key)
     # Check that all the remaining unmatched key-value pairs matches any
     # definition in items or optional_items.
-    for key, value_schema in iteritems(self.optional_items):
+    for key, value_schema in self.optional_items.items():
       if key not in data:
         continue
       value_schema.Validate(data[key])