roll_util: Fix label sorting in Python 3

Bug: 465
Change-Id: I0e54af4db670b00229d756f5393969f324484119
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/63901
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Nathan Mulcahey <nmulcahey@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/recipe_modules/roll_util/api.py b/recipe_modules/roll_util/api.py
index 503e928..f2db5f8 100644
--- a/recipe_modules/roll_util/api.py
+++ b/recipe_modules/roll_util/api.py
@@ -270,7 +270,7 @@
     def __init__(self, props, *args, **kwargs):
         super(RollUtilApi, self).__init__(*args, **kwargs)
         self.labels_to_set = collections.OrderedDict()
-        for label in sorted(props.labels_to_set):
+        for label in sorted(props.labels_to_set, key=lambda x: x.label):
             self.labels_to_set[str(label.label)] = label.value
         self.labels_to_wait_on = sorted(str(x) for x in props.labels_to_wait_on)
         self.footer = []