Fix duplicated exception chain output for exceptions without a traceback

[SemTiOne]

When an exception in a chain has no __traceback__ of its own, repr_excinfo() falls back to traceback.format_exception(), which defaults to chain=True and walks the rest of the chain itself, duplicating what pytest's own chain-walking loop then prints again right after.

Fix: pass chain=False to that fallback call.

Closes #8321

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