Switches to Python 3 support. Made a change to run_code_generation to
support this. Affects several generators. Also updates the generator
to make a few other small cleanups.

Bug: angleproject:5653
Change-Id: I045173c9ca85947c4eac22285701032c09f4c8d0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2699187
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>