Fixed the test Manual/pyobject_related_functions.tst (which was failing for some python builds)

Note: string attribute __doc__ is documented differently in some python versions
e.g. "str(object) -> ..." vs "str(object='') -> ..."
see http://bugs.python.org/review/15516/diff2/5609:6750/Objects/stringobject.c?context=&column_width=80