git-rebase-update: Make tests run on Python 3.

Bug: 1009809
Change-Id: I47c9a468b2922248b823ebae5e71863a698c30f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1889166
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/git_rename_branch.py b/git_rename_branch.py
index 8a07535..d40b606 100755
--- a/git_rename_branch.py
+++ b/git_rename_branch.py
@@ -42,7 +42,7 @@
         if branch_config(branch, 'remote') == '.':
           set_branch_config(branch, 'merge', 'refs/heads/' + opts.new_name)
   except subprocess2.CalledProcessError as cpe:
-    sys.stderr.write(cpe.stderr)
+    sys.stderr.write(cpe.stderr.decode('utf-8', 'replace'))
     return 1
   return 0