This CL adds a new field to the codereview.settings file used by
git_cl for project-wide defaults. If `USE_PYTHON3` is set to True,
then we will run the PRESUBMIT checks under Python3 by default
instead of Python2, unless the PRESUBMIT.py file contains
`USE_PYTHON3 = False` on a line by itself

(as opposed to now, where we'll use Python2 by default unless the
file contains `USE_PYTHON3 = True`).

This will allow us have Python3 be the default for new files
and to eventually eliminate any uses of `USE_PYTHON3` from the
individual presubmit files. Of course, you will have to go in and
explicitly add `USE_PYTHON3 = False` to any Py2-only files prior
to enabling this.

Bug: 1207012
Change-Id: Id8ec45b43940e5bcffeee196398c711c541e733e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2917747Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>