Update the Zenodo downloader to work with the new Zenodo API. Use the
`filename` key for getting the filenames of all files in a repository.
Update the generated download url based on heuristics: the link present
in the API reference doesn't work at the moment). Update the method that
populates the registry: use the new `filename` key and specify that the
checksums are now md5.

Detect which API are we interacting with and generate the download url
and/or populate the registry accordingly.

Test if `ZenodoRepository` correctly supports both the old and new
Zenodo APIs. Use `pytest-httpserver` to run a local server that returns
API response that mimics the Zenodo ones. Define a `base_api_url` class
attribute for `ZenodoRepository` so we can override it in tests.