Update pyproject.toml with Poetry v2.0 (#499)

Migrate from [tool.poetry] to [project]
https://python-poetry.org/blog/announcing-poetry-2.0.0
https://python-poetry.org/docs/pyproject/
https://python-poetry.org/docs/faq/#migrate-pep621-new-static

Update website, changelog urls (#433)

Use dynamic Poetry classifiers and dependencies,
more convenient but less generic to pyproject.toml project

Change classifier to "Operating System :: Unix",
since Silkaj is not supported on Windows

Move pydiscourse to optional-dependencies,
since Poetry extra is deprecated

Set main authors, with full names,
sorted by importance of their contributions

Switch back pip container build to install Poetry v2