Add missing DECREF to PyErr_WriteUnraisable(). That function reports exceptions that can't be raised any further, because (for instance) they occur in __del__ methods. The coroutine tests in test_generators was triggering this leak. Remove the leakers' testcase, and add a simpler testcase that explicitly tests this leak to test_generators. test_generators now no longer leaks at all, on my machine. This fix may also solve other leaks, but my full refleakhunting run is still busy, so who knows?