Move from setuptools to Poetry.

[CSchoel]

The Python tooling ecosystem has involved considerably since I first created nolds. It's time to move to modern tools. Especially, the custom setup.py has to go in favor of a proper pyproject.toml. I also want to use poetry for better dependency management over pip and setuptools.

• Create pyproject.toml.
• Copy all info from setup.py into pyproject.toml (or other respective files, such as MANIFEST.in).
  • Dependencies
  • Specify required versions for dependencies
  • Extras
  • Metadata
• Test installation locally