PHILADELPHIA — Well, that didn’t last long. Democratic elation after John Fetterman came through on his pledge to become the party’s 51st Senate vote — a promise made whole when Sen. Raphael Warnock won reelection in Georgia Tuesday — ran into a speed bump Friday when Sen. Kyrsten Sinema, of Arizona, announced she’s leaving the Democratic Party and becoming an independent. (Pennsylvania has seen this before: The last senator to change parties was Sen. Arlen Specter, who left the GOP to become a Democrat in 2009, only to be replaced by Sen. Pat Toomey, the Republican now being replaced by Fette…