This reverts commit 138ec7c5.

Reason for revert:
This might have caused the following issue:
https://bugs.chromium.org/p/chromium/issues/detail?id=1221233

Original change's description:
> use python3 for goma related scripts
>
> This uses python3 like autoninja{,.bat}.
> https://source.chromium.org/chromium/chromium/tools/depot_tools/+/main:autoninja
> https://source.chromium.org/chromium/chromium/tools/depot_tools/+/main:autoninja.bat
>
> Fixed: 1203012
> Change-Id: I4daf1892024358c5b7d88b1e32b15c82cb4411d2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2968069
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>

Change-Id: Ideb5004cf18c91c1404066622244a8527f5248f3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2972023
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>