To keep build-compatibility with Python 3.6, I've switched the build
backend from `setuptools` to `hatchling`. `setuptools` only supports
`pyproject.toml` from version 61 onwards, which itself isn't compatible
with Python 3.6. This does not mean we have to fully use Hatch as we can
still create build using `python -m build`. Hatchling automatically
includes all non-gitignored files, so the `MANIFEST.in` file has been
removed.