Removing obsolete setup.py and adding pyproject.toml instead

[detrin]

The pyproject.toml file is described in PEP 621. It is the recommended way to package Python projects these days. On the other hand, setup.py is obsolete, but not yet deprecated. More on the migration in https://packaging.python.org/en/latest/guides/modernize-setup-py-project/

If I have spare time, I will open a PR.