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/mb/mb.py

@@ -919,10 +919,10 @@ class MetaBuildWrapper(object):
 
     cmdline = []
     extra_files = [
-        '../../.vpython',
+        '../../.vpython3',
         '../../testing/test_env.py',
     ]
-    vpython_exe = 'vpython'
+    vpython_exe = 'vpython3'
 
     must_retry = False
     if test_type == 'script':