SkQP bash scripts: use python3

This restores the ability to run these scripts now that we should all be
using python3.

Change-Id: Ia43bea81f904932d3dc0f733c41f5ca7a9ac11f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/744976
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed