[py3] Migrate native_client_sdk/src/doc PRESUBMIT

Bug: 1212048
Change-Id: Ie72b8db6c6003d3cb3f36ea84c51fb3420a06cd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2957266
Reviewed-by: Sam Clegg <sbc@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Cr-Commit-Position: refs/heads/master@{#891870}

native_client_sdk/src/doc/PRESUBMIT.py

@@ -4,6 +4,9 @@
 
 import subprocess
 
+USE_PYTHON3 = True
+
+
 def _CheckSphinxBuild(input_api, output_api):
   """Check that the docs are buildable without any warnings.
 
@@ -18,7 +21,7 @@ def _CheckSphinxBuild(input_api, output_api):
                             stderr=subprocess.STDOUT)
   except subprocess.CalledProcessError as e:
     return [output_api.PresubmitNotifyResult('sphinx_build failed:\n' +
-            e.output)]
+                                             e.output.decode('utf-8'))]
 
   return []