SPRINGFIELD — The Illinois House voted Wednesday to endorse a proposed U.S. constitutional amendment that would finally eliminate what some see as the last vestige of slavery in the United States — forced labor by people convicted of crimes and sentenced to prison.House Joint Resolution 7, by Rep. Mary Flowers, D-Chicago, calls for passage of the proposed “abolition amendment,” which seeks to eliminate what’s known as the “punishment clause” of the 13th Amendment, which abolished slavery after the Civil War.The 13th …