Package metadata is now in `pyproject.toml` instead of `setup.py`.
Package now uses src folder layout.
All classes and functions are now documented.
All functions are type hinted.
Mypy shows no type errors.
Pylint only shows errors about different arguments in child class of `Helper` abstract class `helper` method.
`py.typed` file added.
Test code is removed from package files and put in `example` directory.
Add license file.
`__init__.py` cleaned up.