Updating scripts/txt_to_json.py to run with guard

[AaronCritchley]

Python scripts should run 'guarded', meaning that they only execute when you intend, and not when importing or scanning (like when executing doctests).

This can cause issues if people bring blns into their repo as a submodule, as things like test runners or doctest runners will have to manually exclude this file when they are running, which is inconvenient.

This stack overflow answer gives more detail on why this is important.

p.s. this script doesn't actually work for me, does it have tests somewhere?