Update swh.storage.validate for swh.model 0.7.2
swh.model.model.ModelObject.compute_hash was changed to a method instead of a staticmethod.
Of course, now that I notice that it was used elsewhere than swh.model, this would have warranted a swh.model 0.8.0...
Test Plan
tox tests pass again
Migrated from D4362 (view on Phabricator)