The changes are to allow build files to be generated using Python 3 on Windows.

The scripts still work with Python 2.
There are no intended behaviour changes.

Bug: 941669
Change-Id: Ia9d8d61373ee37b4470fd5479e9db01e17994ffa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1520649
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>