pyproject.toml as single source for dependencies?

Hi,

Would you be open to a pull request that unifies the dependencies into pyproject.toml? It would make dependencies easier to maintain, but remove the conda installation option.

For context, we recently did exactly that in our own [fork](https://github.com/esciencecenter-digital-skills/scikit-learn-mooc) of this course. The diffs of the relevant pull request are here: https://github.com/esciencecenter-digital-skills/scikit-learn-mooc/pull/61/changes. If that's useful for you, I can prepare a PR.

Thanks!