The problem was in the following code:
class ArteTVPlus7IE(ArteTVBaseIE):
...
@classmethod
def suitable(cls, url):
return False if ArteTVPlaylistIE.suitable(url) else super(ArteTVPlus7IE, cls).suitable(url)
And its sublcasses like ArteTVCinemaIE.
Since in the lazy_extractors.py file ArteTVCinemaIE was not a subclass of ArteTVPlus7IE, super(ArteTVPlus7IE, cls) failed.
To fix it we have to make it a subclass. Since the order of _ALL_CLASSES is arbitrary we must sort them so that the base classes are defined first. We also must add base classes like YoutubeBaseInfoExtractor.
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