Refactor pyproject.toml for Poetry v2

We keep the python version constraint and runtime Python dependenices in
`tool.poetry.dependencies` and mark `dependencies` and `requires-python`
as dynamic in the `[project]` section.

We don't switch to `project.requires-python` since Renovate is already
configured to not touch that constraint, and just from the docs I'm not
sure if/how we can disable renovate for `project.requires-python`.

Additionally, we don't switch to `project.dependencies` since we'd have
to rewrite all dependency definitions to PEP621-compatible strings.

Author: simu