Migrate to new project structure `pyproject.toml`

[Rotzbua]

This issue is to collect resources and plan the migration.

Reason

The old installation with setup.py is deprecated.

Reference

• https://packaging.python.org/en/latest/specifications/
• https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/
• https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html

[florianfesti]

This probably needs a second look.