Anthony Weiner is trying to transform himself into the next Longfellow. The serial sexter and former congressman — who is now living in a Bronx halfway house after doing hard time for sexting a teenager — is skulking around Manhattan’s publishing houses trying to shop a book proposal, literary sources say. So far, interest has been flaccid. “Every Simon & Schuster imprint has passed,” an insider told The Post. The project is being repped by the hot downtown boutique agency Foundry Literary + Media — which was behind nonfiction best-sellers by “Daily Show” host Trevor Noah, New Orleans Saints coach...