fix async generator bug when yielding a StopIteration

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-619 into branch/py3.6

Created originally on Bitbucket by raduciorba (Radu Ciorba)

Was already merged in Bitbucket before import, marked as merged by the import user

Make sure we raise an instance of StopIteration, not the value, so we don’t need normalize_exception later.

This should fix:

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