Python 3 support.

In Python 3.3, `inspect.trace()` returns an empty list which causes another
exception to be thrown. The source for `--pdb` in nose merely calls
`pdb.post_mortem(tb)`.  This function is, more or less,

    p = pdb.Pdb()
    p.reset()
    p.interaction(None, tb)

This seems to fix the bug. In addition, the frame where the exception was
thrown does seem to be found anyway as all of the local variables are
accessible.