Run custom git commands (e.g. git-cl, git-new-branch) using Python 3
when GCLIENT_PY3 is set to 1.

Bug: 984182
Change-Id: I81635b6cb83b2e0945d6596a7dc363f702075466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2079704Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>