This reverts commit 59140d43.

Reason for revert:
Make this works in git bash.

Original change's description:
> Revert "use python3 for goma related scripts"
>
> 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
> >
> > 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>

Bug: 1221233
Change-Id: I815a774faa2e4fbca0f262667ea89709a09e146b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2979560
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>