Fix three Python reference leaks in PLy_traceback(). This would result

in leaking memory when invoking a PL/Python procedure that raises an
exception. Unfortunately this still leaks memory, but at least the
largest leak has been plugged.

This patch also fixes a reference counting mistake in PLy_modify_tuple()
for 8.0, 8.1 and HEAD: we don't actually own a reference to `platt', so
we shouldn't Py_DECREF() it.