Thanks, @cvp — I tried that, with or without the query string. I also tried adding root=icloud since that’s where the script is. I tried it prefixed with pythonista:/// per the docs, prefixed with a leading slash, and prefixed with nothing as you show above.
The script is in an iCloud folder, so I’ve tried:
/folder/script.py?action=run&root=icloud
folder/script.py?action=run&root=icloud
pythonista3:///folder/script.py?action=run&root=icloud