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Major:
- Turn ``av.ValueError`` into ``av.ArgumentError``. The latter is now not a subclass of ``ValueError``. This change better reflects how users should think about this exception.
- Make ``SubtitleStream.decode()`` return the list of subtitles directly, without the intermediate ``SubtitleSet``.
- Drop Support for Python 3.9.
Features:
- Add support for Python 3.13t.
- Add ``SubtitleCodecContext.decode2()`` which returns ``SubtitleSet | None``.