Remaining fallout from 17911

Remaining fallout from 17911

The code module was using a private function from traceback in order to skip a
frame - used the direct interface to do that instead,

The decimal module suffered minor fallout from formatting changes ('None' as a
value is now not printed by traceback, the same as None was not before).

The cgitb module was passing a bogus exception type (type.__name__) into
format_exception, which uncovered that format_exception and print_exception had
been ignoring the etype for some time, so the compatibility thunk to the new
code now does the same thing.