‘Avengers: Infinity War’ opens tonight. There are 64 superhero characters in the film. In this clip from the Jimmy Fallon Show, Robert Downey Jr., Scarlett Johansson, Chris Hemsworth, Chris Evans, Chris Pratt, Anthony Mackie, Benedict Cumberbatch, Chadwick Boseman, and Tom Hiddleston sing ‘The Marvel Bunch’. It’s a parody of the Brady Bunch Theme…