Near the end of FX’s TV adaptation of “Fleishman Is in Trouble,” after the upscale lives of its privileged main characters have emerged from their slow-motion implosion, one character tells another character about the book she might write about the whole mess. “What will it be about?” her friend asks. “It will be about everything,” she says. “That’s heavy,” he replies. And so it is. Based on Taffy Brodesser-Akner’s bestselling novel of the same name, “Fleishman Is in Trouble” — which debuted on Hulu this week — is about the reckoning that comes with adulthood. It is about the challenges of mar…
