PHILADELPHIA — Pennsylvania’s crucial U.S. Senate race has been the most expensive in the country this year — and it’s not even close. Democrat John Fetterman, Republican Mehmet Oz and their political allies have spent a combined $312 million on a race that could determine control of the Senate, according to OpenSecrets, a nonpartisan group that tracks money in politics. And that’s not counting Pennsylvania’s costly Senate primary. Add the receipts from May and the tab rises to close to $374 million. The eye-popping cost reflects both the high stakes of Pennsylvania’s race, and the cost of adv…