Support fsspec 2023.1.0 in CI
Support fsspec 2023.1.0 in CI.

In the 2023.1.0 fsspec release, they replaced the type of `fsspec.registry`:
- from `ReadOnlyRegistry`, with an attribute called `target`
- to `MappingProxyType`, without that attribute

Consequently, we need to change our `mock_fsspec` fixtures, that were using the `target` attribute.

Fix #5448.