Reland "Add Python 3 support to GetPythonUnitTests"

This reverts commit c03ebf0e.

Reason for revert: the original change was not the cause of the test
failures. The actual cause was crrev.com/c/3683105 which was reverted.

Original change's description:
> Revert "Add Python 3 support to GetPythonUnitTests"
>
> This reverts commit dfc71bbe.
>
> Reason for revert: Breaks all presubmits that do not have skip_shebang_check=True set.
>
> Original change's description:
> > Add Python 3 support to GetPythonUnitTests
> >
> > GetPythonUnitTests (and by extension RunPythonUnitTests) always ran
> > scripts using Python 2. This change adds a python3=False parameter to
> > GetPythonUnitTests to give the option of switching to Python 3, while
> > leaving the default as Python 2. The child scripts are run under one or
> > the other, based on this parameter.
> >
> > This change is needed in support of crrev.com/c/3679801.
> >
> > Bug: 1313804
> > Change-Id: Ic59287352d4941707adaf7981ed7af4201b7d526
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3680099
> > Reviewed-by: Jesse McKenna <jessemckenna@google.com>
> > Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
>
> Bug: 1313804
> Change-Id: I1bd7096b6cfdfed6205e6dc0b5d4498b5b821b97
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3683378
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Commit-Queue: Christoffer Jansson <jansson@chromium.org>
> Owners-Override: Ben Pastene <bpastene@chromium.org>
> Reviewed-by: Tomas Gunnarsson <tommi@chromium.org>

Bug: 1313804
Change-Id: I15ed90cc35d88ece71eb681acc0acc06cb5c3bcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3685229
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>

presubmit_canned_checks.py

@@ -883,7 +883,7 @@ def GetUnitTestsRecursively(input_api,
                       skip_shebang_check=skip_shebang_check)
 
 
-def GetPythonUnitTests(input_api, output_api, unit_tests):
+def GetPythonUnitTests(input_api, output_api, unit_tests, python3=False):
   """Run the unit tests out of process, capture the output and use the result
   code to determine success.
 
@@ -922,7 +922,10 @@ def GetPythonUnitTests(input_api, output_api, unit_tests):
         backpath.append(env.get('PYTHONPATH'))
       env['PYTHONPATH'] = input_api.os_path.pathsep.join((backpath))
       env.pop('VPYTHON_CLEAR_PYTHONPATH', None)
-    cmd = [input_api.python_executable, '-m', '%s' % unit_test]
+    if python3:
+      cmd = [input_api.python3_executable, '-m', '%s' % unit_test]
+    else:
+      cmd = [input_api.python_executable, '-m', '%s' % unit_test]
     results.append(input_api.Command(
         name=unit_test_name,
         cmd=cmd,