MILAN (Reuters) -Italy’s Agnelli family, whose investments include carmaker Stellantis and soccer club Juventus, is set to buy a slice of French healthcare group Institut Merieux for 833 million euros ($867 million) through its Exor holding company. The deal, announced on Friday by the two companies, will see privately-held Institut Merieux selling a 10% stake to Exor through a reserved capital increase. It marks the first major international move into the healthcare business by Exor, which will soon have 9 billion euros to invest after it completes the sale of Bermuda-based reinsurer Partner …