(C5) setting f_lineno didn't evolve when the rest of the bytecodes evolved,
  which means it is not safe any more::

    import sys

    def f():
        try:
            raise ValueError    # line 5
        except ValueError:
            print(42)           # line 7

    def my_trace(*args):
        print(args)
        if args[1] == 'line':
            f = args[0]
            if f.f_lineno == 5:
                f.f_lineno = 7
        return my_trace

    sys.settrace(my_trace)
    f()
    sys.settrace(None)