Article 6. there shall be neither slavery nor involuntary servitude in the said territory, otherwise than in the punishment of crimes whereof the party shall have been duly convicted: provided, always, that any person escaping into the same, from whom labor or service is lawfully claimed in any one of the original states, such fugitive may be lawfully reclaimed and conveyed to the person claiming his or her labor or service as aforesaid. … — northwest ordinance, 1787 in which supreme court case did this provision of the northwest ordinance play an important part?