message: mb: decode process output for Python3
  CL from chromium's mb:
  Cr-Commit-Position: refs/heads/main@{#36244}
diff: @@ -1213,6 +1213,8 @@ class MetaBuildWrapper:
  stderr=subprocess.PIPE,
  env=env)
  out, err = p.communicate()
  + out = out.decode('utf-8')
  + err = err.decode('utf-8')
  else:
  p = subprocess.Popen(cmd, shell=False, cwd=self.src_dir, env=env)
  p.wait()
  