Replace setuptools with pyproject.toml

[cdorsman]

According to official Python documentation, setuptools is replaced in favor of pyproject.toml to define and package a Python project.

In case of this project, transitioning from setuptools to pyproject.toml: No more maintaining three files: requirements-dev.txt, setup.cfg and setup.py, but one file, pyproject.toml where everything is stored.

This means easier maintenance

[grimley517]

In the existing pyproject.toml file I notice that there is a license reference to MIT. However there is no license associated with this project.

@faif
is MIT the correct license for this project, or are you intending to use another model?

[faif]

@grimley517 see #143

[grimley517]

Thanks @faif I was aware of that. That's why I removed the license in the PR that you approved. Can this issue be closed per PR #446?