This is a reland of bb88492f

Original change's description:
> Use py3 in bot_update and gclient recipe_modules
>
> Recipe-Nontrivial-Roll: build
> Recipe-Nontrivial-Roll: build_limited
> Recipe-Nontrivial-Roll: chrome_release
> Recipe-Nontrivial-Roll: chromiumos
> Recipe-Nontrivial-Roll: infra
> Bug: 1289280
> Change-Id: Id61bacf4acef319f7ea22161081b632677ca363c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3402082
> Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited
Recipe-Nontrivial-Roll: chrome_release
Recipe-Nontrivial-Roll: chromiumos
Recipe-Nontrivial-Roll: infra
Bug: 1289280, 1289504
Change-Id: I2f2f7244568b1fdfb960d5cdfa3937175ac1ed2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3414742Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>