Update PRESUBMIT.py to python3.

Change-Id: Id99f132b1a8c9202f6ace7f0582319f8388c881a
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 87fd416..b916f47 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -111,6 +111,7 @@
 def CheckChange(input_api, output_api):
   branch = (
       check_output(['git', 'cl', 'upstream'])
+          .decode('utf-8')
           .strip()
           .replace('refs/heads/', ''))
   results = []