Restore postmortem debugging

[aecay]

It turns out that there are differences between set_trace and
post_mortem debugger calls. Unfortunately PEP553 doesnʼt support
customizing the latter, so we have to DIY.

[paulpepper-sky]

With side-channel explanation from Aaron, makes sense to default to using pdb.post_mortem() as a way to enter the debugger at the stack frame from the exception, rather than at the point where the breakpoint() is called.