testing: support python3 in headless_python_unittests.py for M104

(cherry picked from commit d0a8c00a16849a549b80ec49146f5c7db42586df)

Bug: 1340449
Change-Id: I364fce6f0b86f0129e8f9e94de744c59dea115ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3734707
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1019917}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3739620
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/branch-heads/5112@{#642}
Cr-Branched-From: b13d3fe7b3c47a56354ef54b221008afa754412e-refs/heads/main@{#1012729}

diff --git a/headless/lib/browser/devtools_api/client_api_generator_unittest.py b/headless/lib/browser/devtools_api/client_api_generator_unittest.py
index 9e1ca65..cd49df0 100755
--- a/headless/lib/browser/devtools_api/client_api_generator_unittest.py
+++ b/headless/lib/browser/devtools_api/client_api_generator_unittest.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
@@ -15,7 +15,7 @@
 
   def test_ArgumentParsing(self):
     with tempfile.NamedTemporaryFile() as f:
-      f.write('{"foo": true}')
+      f.write(b'{"foo": true}')
       f.flush()
       json_api, output_dir = client_api_generator.ParseArguments([
           '--protocol', f.name, '--output_dir', 'out'])

diff --git a/testing/scripts/headless_python_unittests.py b/testing/scripts/headless_python_unittests.py
index 4e32907..cb0bb375 100755
--- a/testing/scripts/headless_python_unittests.py
+++ b/testing/scripts/headless_python_unittests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -38,7 +38,7 @@
   valid = bool(rc <= common.MAX_FAILURES_EXIT_STATUS and
                ((rc == 0) or failures))
   common.record_local_script_results(
-      'headless_python_unittests', args.output, failures.keys(), valid)
+      'headless_python_unittests', args.output, list(failures.keys()), valid)
 
   return rc