[py3] Decode stdout in presubmit runs

Bug: 1215375
Change-Id: I068801f5e7482cf13cf70dd6c3a70c39f22a39f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2949974
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

presubmit_support.py

@@ -208,7 +208,7 @@ class ThreadPool(object):
       stdout, _ = sigint_handler.wait(p, stdin)
       if timer.completed:
         stdout = 'Process timed out after %ss\n%s' % (self.timeout, stdout)
-      return p.returncode, stdout
+      return p.returncode, stdout.decode('utf-8', 'ignore');
 
   def CallCommand(self, test):
     """Runs an external program.