The moto@v5 do not implement mock_s3 anymore, it uses mock_aws instead.
This version is used since we do have a python3.9 project (under it uses a
moto@v4 in which mock_s3 is still present).

Instead of doing storage attribution in test setup, we implement a test
hook (in test/data/hook.py) as would be done in real application. As a
consequence, we change the way s3 requests are mocked (using moto).
Namely, we introduce a "_s3_client" class method in S3Storage that is
mocked in test setup so that mocking operates in the test hook.

In migration tests, instead of instantiating a new storage object, we
reuse the one bound to Image's data attribute so as to benefit from the
active mock.

So as to ease testing in downstream projects.