Some suggestions

1. Add __init__py to the tests directory for test files be able to import jsonflattifier module when running pytest.
2. Change relative path in all test-cases json files to start from tests directory.
3. Add terminaltables module into Pipfile and Pipfile.lock.
4. Use f-strings for string manipulations.
5. Add json_load_string and json_load_file functions into helper.py.
6. Add __rpocess_data and __cartesian_product functions into helper.py.
7. Delete excess code from __denormalise function.