Fix `up` through generators in postmortem debugger By passing the lowest traceback element into the debugger, we require it to use frame.f_back to find older frames. However, f_back is always None for generator frames. By providing a higher-up traceback element, pdb can traverse down the traceback.tb_next links, which do work correctly across generator calls. Furthermore, pdb will not do the right thing if it is provided with a frame/traceback pair that do not correspond to each other - it will duplicate parts of the callstack. Instead, we provide None as a frame, which will cause the debugger to start at the deepest frame, which is the desired behavior here.