Gene Maddaus Senior Media WriterOn Tuesday, “Veep” showrunner David Mandel will sit down for a panel with two other writers, Etan Cohen (“Idiocracy”) and Eli Attie (“The West Wing”), for a conversation entitled “Screenwriting the White House.”It sounds like the sort of virtual roundtable that somebody like, well, Variety, would do. But this is actually a fundraiser for Joe Biden’s presidential campaign.