Migrate from deprecated setup.py to pyproject.toml

[mstoelzle]

This PR fixes the following deprecation warning in PIP 25.1

DEPRECATION: Building 'normalizing-flows' using the legacy setup.py bdist_wheel mechanism, which will be removed in a future version. pip 25.3 will enforce this behaviour change. A possible replacement is to use the standardized build interface by setting the `--use-pep517` option, (possibly combined with `--no-build-isolation`), or adding a `pyproject.toml` file to the source tree of 'normalizing-flows'. Discussion can be found at https://github.com/pypa/pip/issues/6334

by migrating the package from a setup.py to a pyproject.toml.