WASHINGTON — In recent months, Sen. Bernie Sanders’ political team has noticed a marked shift in the 80-year-old former presidential candidate: His campaign fires are burning hot. On Thursday, Sanders, I-Vt., will travel to Pittsburgh to headline a rally for Summer Lee, one of a half-dozen hard-core progressives he has endorsed in contested Democratic congressional primaries. He also plans to meet with Starbucks workers in the city to show solidarity in a store-by-store unionization push.