An AT&T retail schedule obtained by 9to5Mac hints at a Sept. 23 release date for the new iPhone. Previous rumors predicted pre-orders for the new phone will start on Sept. 9, with a likely release date of Sept. 16. The leaked schedule shows AT&T highlighting extra hours needed for a “merchandising rest” on both Sept. 9 and Sept. 23. Either is a possibility: The wait between the start of pre-orders and the official release date…