Partially upgrade codegen to Python 3 Bug: angleproject:5707 Change-Id: I4e26c2c7ba4a36185b28ecd6e87be1161b60431a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2729567 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>