This experimental patch proposes to defer generator cleanup to the frame itself. In this scheme, the generator frame's tp_clear throws the GeneratorExit if necessary, so as to call cleanup code. The generator doesn't have any tp_del anymore, as it is now impossible to resurrect a generator (the frame, though, can be resurrected; I have to add a test for that).

The net effect is that generators caught in a reference cycle can always be reclaimed, and their cleanup code is run in a valid frame.