Essentials

- Drop Python 3.6 and Python 3.7 support.
- Use `obj.model_dump` instead of `obj.dict` for Pydantic V2.
- Add support for `Decimal` in `FriendlyEncoder`.
- Apply `black`, `flake8`, `isort`, `mypy`.
- Replace `setup.py` with `pyproject.toml`.
- Workflow maintenance.

- Add `py.typed` file.
- Add `ConflictException`.

- Migrate to GitHub Workflows.
- Improve build to test Python 3.6 and 3.9.
- Improve the `json.FriendlyEncoder` class to handle built-in `dataclasses`.
- Add a changelog.
- Improve badges.