Switch Swarming commands to py3.

Thanks to machenbach@ for the heads up.

WebRTC's MB was generating a Swarming command that was using vpython
(defaulting on python2 on some platforms). This CL switches that to
vpython3 (fixing gtest-parallel-wrapper.py to be python3 compliant).

No-Presubmit: True
Bug: webrtc:13607
Change-Id: Icfa7d23b81e30cebfe8243d4ba65284955593465
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/249604
Reviewed-by: Christoffer Jansson <jansson@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35821}

tools_webrtc/gtest-parallel-wrapper.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 
 # Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
@@ -226,7 +226,7 @@ def main():
   if test_artifacts_dir and not os.path.isdir(test_artifacts_dir):
     os.makedirs(test_artifacts_dir)
 
-  print 'gtest-parallel-wrapper: Executing command %s' % ' '.join(command)
+  print('gtest-parallel-wrapper: Executing command %s' % ' '.join(command))
   sys.stdout.flush()
 
   exit_code = subprocess.call(command, env=test_env, cwd=os.getcwd())