pytest_plugin: Ensure swh_storage_backend backward compatibility
Merged
requested to merge anlambert/swh-storage:retry-proxy-register-partitions-interface into master
Recent changes in swh-storage added the PartitionsManagementInterface
but this interface is only implemented on real storage backends, not
proxies.
Any client code overriding the swh_storage_backend_config to return a
storage pipeline (loaders for instance) ends up with error otherwise.