PHILADELPHIA — He rallied in Erie and marched in Pittsburgh. He held a town hall at a rural inn in Franklin County, spoke to union members in the Lehigh Valley, and headlined Democratic dinners in Northeastern Pennsylvania. But since launching his campaign in February 2021, Democratic Senate candidate John Fetterman hasn’t held a public event in Pennsylvania’s biggest, bluest city. Until now. On Saturday, less than 50 days before a general election that could decide control of the U.S. Senate, Fetterman will hold his first open event in Philadelphia as he rallies with U.S. Rep. Dwight Evans. I…