PRESUBMIT.py scripts are executed in presubmit_support.py using exec().
Since PRESUBMIT.py scripts might not be yet compatible with python 3, we
have to execute presubmit_support.py using python 2.

git_cl.py imports presubmit_support.py, and executes presubmit checks using
presubmit_support as a module. This forces git_cl.py to be executed using
python 2 to maintain compatibility for PRESUBMIT.py scripts.

This change allows git_cl.py to be executed using python 3, while
presubmit_support.py is executed using python 2.

Similar changes for post-submit hooks and git-cl try masters will follow.

Bug: 1042324
Change-Id: Ic3bb1c2985459baf6aa04d0cc65017a1c2578153
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2068945Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>