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: http://buildbot.pypy.org/summary/longrepr?testname=unmodified&builder=pypy-c-app-level-linux-x86-64&build=3968&mod=lib-python.3.test.test_asyncgen