allow __suppress_context__ and __notes__ to be mutated on frozen exceptions

[graingert]

Summary

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[graingert]

from pep678 https://peps.python.org/pep-0678/#specification

Libraries may clear existing notes by modifying or deleting the notes list, if it has been created, including clearing all notes with del err.notes. This allows full control over the attached notes, without overly complicating the API or adding multiple names to BaseException.dict.

[hynek]

Thanks!