Python dataset support ubuntu 19/20

Issue: When attempting to download a dataset on ubuntu 19.10 I get `NotImplementedError: Unsupported Linux distribution ubuntu 19.10`.

It seems like the problem is that the dotnetcore2 pip package actually only supports ubuntu 18. But ubuntu 20.04 is the new LTS, so it makes sense to support it (and also ubuntu 19).

Also, can we agree that it is a bit of an architecture-smell when downlading some csv-files (the dataset) causes a dependency to go look for a distro-specitic tar-file for a custom installation of a third dependency? I don't know whats the best solution, but this cant be it.

Related: https://github.com/Azure/MachineLearningNotebooks/issues/713