Update pyproject.toml to PEP standard

[another-rex]

Now that poetry has released v2.0.0, we can switch all of our pyprojects.toml to support the PEP standard.

This includes

• Updating both the top section (name, description...), see here:
  https://python-poetry.org/blog/announcing-poetry-2.0.0/#supporting-the-project-section-in-pyprojecttoml-pep-621
• Updating how dependencies are specified, see: https://python-poetry.org/docs/dependency-specification/
• Updating our documentation for how to install poetry v2 (use pipx?), removal of poetry shell (use poetry env activate instead)...etc

Things still to figure out:

• Renovatebot support for the new format