qa: Fix python TypeError in script.py

Summary:
Backport of core PR12475
https://github.com/bitcoin/bitcoin/pull/12475/files

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3521