GH-100964: Fix reference cycle in exhausted generator frames

[savannahostrowski]

So after spelunking here it appears that the issue here is that we can't clear frame->previous after _PyFrame_ClearExceptCode in clear_gen_frame because _PyFrame_ClearExceptCode calls take_ownership which copies the frame with a stale previous pointer. This makes f_back point to the caller, instead of None.

• Issue: Strange reference cycle (?) introduced with shim frames #100964