For those who would argue that Stephen Sondheim’s lyrics are too clever by half, “A Little Night Music” could be Exhibit A in their speciousness. The 1973 musical — inspired by the Ingmar Bergman film “Smiles of A Summer Night,” and featuring a book by Hugh Wheeler, also Sondheim’s collaborator on “Sweeney Todd” — focuses on the romantic follies of a well-off set in turn-of-the-century Sweden, circa 1900.