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.

IPython/core/ultratb.py

@@ -1203,7 +1203,7 b' class VerboseTB(TBTools):'
                 if etb and etb.tb_next:
                     etb = etb.tb_next
                 self.pdb.botframe = etb.tb_frame
-                self.pdb.interaction(self.tb.tb_frame, self.tb)
+                self.pdb.interaction(None, etb)
 
         if hasattr(self, 'tb'):
             del self.tb