This reverts commit 01ed3589.

Reason for revert: Broke tools on Windows with bash

Original change's description:
> depot_tools: Run using Python 3 by default.
> 
> Run gclient, roll-dep, fetch and custom git commands (i.e. git-cl, git-rebase-update, git-new-branch, etc.)
> using vpython3 by default.
> 
> Change-Id: I4eecddafa6ca4c5f82ec097615c79d2a741613e7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2113550
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,ajp@chromium.org,apolito@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: If29ad72feaa215b8b282f2f6c9911700dd2f228c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2116728Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>