Hash stability failure in `Dataset` with newer `numpy`

[sirmarcel]

test_hash_stable in tests/test_dataset.py has started failing when numpy is upgraded somewhere above 1.16.5. Most likely this is due subtly different treatment of object arrays. Luckily, they should become obsolete once we transition to the new Data backend, so I'll close this issue then. Until then, it'll remain open in case anyone else notices!

[sirmarcel]

Hm, newer versions of joblib may also be the culprit.