Move `setuptools` configuration into `pyproject.toml`

Since version 61.0 (March 24th 2022) setuptools now [supports `pyproject.toml`](https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html) and its docs advertise `pyproject.toml` as the first/preferred place for setuptools config. If we move the setuptools config from `setup.cfg` into `pyproject.toml` then if we also remove the `pycodestyle` config from `setup.cfg` (https://github.com/hypothesis/cookiecutters/issues/52) we could get rid of the `setup.cfg` file entirely and have one less root file.