Upgrade overlay fonts generator to Python 3 Bug: angleproject:5707 Change-Id: I809669d7a7de16ed2e098f59067c8e1d9c44a75c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2773292 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>