Parse bounded JSON and XML invoices safely: reference solution

Open this only after you have attempted the repair in the exercise download.

1. Extract this archive into the folder where you extracted bounded-data-parsing.zip.
   It adds reference.py and this file, and changes none of your files.
2. Follow the reference commands in that folder's README.txt; they select the
   reference with EXERCISE_MODULE=reference or a --reference option.
3. Compare reference.py with your exercise file. A different repair can still be
   correct: the tests, not the reference text, define the required behavior.
