Add python3 support to gsutil recipe module

Bug: 1227140
Change-Id: I2a27289335c66f34adaa47be1e9fd0fd13abcc56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3010716
Commit-Queue: Anthony Polito <apolito@google.com>
Reviewed-by: Anthony Polito <apolito@google.com>

recipes/recipe_modules/gsutil/api.py

@@ -162,7 +162,7 @@ class GSUtilApi(recipe_api.RecipeApi):
   def _generate_metadata_args(self, metadata):
     result = []
     if metadata:
-      for k, v in sorted(metadata.items(), key=lambda (k, _): k):
+      for k, v in sorted(metadata.items(), key=lambda k: k[0]):
         field = self._get_metadata_field(k)
         param = (field) if v is None else ('%s:%s' % (field, v))
         result += ['-h', param]