Update gn_to_bp_utils to support python3

Python2 is going away, so update the script to run with python3.

Change-Id: I8c1aad12ecf8ae1bde67a1ad5486bf6feb1c04d1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436098
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
1 file changed