[issue28893] Make sure exceptions raised in __aiter__ are properly chained in ceval

[Yury Selivanov]

Yury Selivanov added the comment:

Ned, sounds good.

Serhiy, we don't have _PyErr_FormatFromCause in 3.5, so I'll probably commit the patch as is in 3.5.  For 3.6 is definitely makes sense to use _PyErr_FormatFromCause.

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