This is a reland of e656e988

Original change's description:
> gclient: Add a GCLIENT_PY3 env var to allow users to execute gclient using Python 3.
> 
> Bug: 1008202
> Change-Id: I23dd70b72089a09c2a7cdc74e2a63c135a924ebe
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1825996
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1008202
Change-Id: I5181a7f8efb39bdd6adc99db90663cf3cf3cd0bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1828334Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>