Likely not the intended answer, as this does not fit any of the hints in the question, but it is a built-in module that
has two different functions that return something printable when given 'Hello world.' as their first argument.
I've given up and tried to bruteforce it - with little regards to what could happen to my machine.
I'm importing every builtin module and evaluating the input on every function they've got. As I don't know a list of builtins, I'm using the help("modules") one, the pkgutil.iter_modules() one, and the sys.builtin_module_names one.