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>