This currently returns a photo album for each shared library. This can then be
used to iterate over all the photos in that library. For example:

    for library_name, album in api.photos.libraries.items():
        print(f'Library: {library_name}')
        for photo in album:
            print(f'{photo.asset_date} {photo} {photo.filename}')

The libraries are now returned as a new object type (PhotoLibrary). That
provides access to albums. This way the icloud-photos-downloader can also use
the Recently Deleted special album to sync deletions.

The list of non-special albums is currently returned empty for the
shared libraries. This also tracks with the fact that the iCloud web app
currently doesn't provide access to albums in shared libraries.