Update downloading from MinIO to accommodate for new structure

[PGijsbers]

Previously, each dataset had their own bucket: https://openml1.win.tue.nl/datasets61/dataset_61.pq

But we were advised to reduce the amount of buckets and
favor hosting many objects in hierarchical structure, so
we now have instead some prefixes to divide up the
dataset objects into separate subdirectories: https://openml1.win.tue.nl/datasets/0000/0061/dataset_61.pq

I started work on it here: /~https://github.com/openml/openml-python/tree/fix/new_minio
It works but is ugly and I didn't run any tests.
Just trying to get it to work for now so Taniya and Prabhant can continue on with their deep learning integration, but we need to integrate this in the next release.

[PGijsbers]

Jos is currently working on bringing minio and parquet to the test server. I assume that will also have the "new style" buckets and object prefixes. @josvandervelde is this correct, and can you give a ping here when that is done?

@eddiebergman @LennartPurucker it would be great if one of you could pick this up when Jos has indicated there's minio/parquet on the test server.