Adds a .yapfignore file so that Python files can be excluded from
"git cl format". This currently only ignores
scripts/export_targets.py since its use of Python 3 causes issues.

This requires depot_tools revision
59b06a838a4e06c43075db252921f4bd45f7f673 or later to work, as
.yapfignore files were unused until then.

Bug: angleproject:3985
Change-Id: Ib9e10f6b5d6baf6b87e7edd0525885553508829d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1860319
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>