[jorenham]

This fixes the following stubtest errors:

av.subtitles.stream.SubtitleStream cannot be subclassed at runtime, but isn't marked with @final in the stub
av.subtitles.stream.SubtitleStream.decode2 is not present at runtime

av.subtitles.subtitle.AssSubtitle cannot be subclassed at runtime, but isn't marked with @final in the stub
av.subtitles.subtitle.BitmapSubtitle cannot be subclassed at runtime, but isn't marked with @final in the stub
av.subtitles.subtitle.BitmapSubtitle is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.subtitles.subtitle.BitmapSubtitlePlane cannot be subclassed at runtime, but isn't marked with @final in the stub
av.subtitles.subtitle.BitmapSubtitlePlane is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.subtitles.subtitle.Subtitle is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.subtitles.subtitle.SubtitleProxy is not present in stub
av.subtitles.subtitle.SubtitleSet cannot be subclassed at runtime, but isn't marked with @final in the stub
av.subtitles.subtitle.SubtitleSet is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.subtitles.subtitle.SubtitleSet.__getitem__ is inconsistent, stub parameter "i" should be positional-only (add "/", e.g. "key, /")

One stubtest error I left unresolved, because this decode2 method appears to actually be used in the tests:

av.subtitles.stream.SubtitleStream.decode2 is not present at runtime