WASHINGTON — A panel of 11 appeals court judges on Tuesday seemed reluctant to overturn a ban on U.S. government contractors donating money to federal elections, a prohibition that has been on the books for 70 years. A lawyer and two government retirees sued in an effort to topple the restriction, which carries a five-year prison term for violations. The three are a university law professor who holds a $12,000 federal contract and two retirees working for the U.S. Agency for International Development as …