Attention, o ye members of the cult of Stephen Sondheim! The Roundabout Theatre’s musical theatre producing arm will benefit from a Dec. 3 reunion concert of the company’s 2004 Tony Award-winning revival of Sondheim and John Weidman’s “Assassins.” Neil Patrick Harris, Denis O’Hare, Marc Kudisch, Becky Ann Baker and Michael Cerveris are among the returning troupers; the full cast will be back. That production was originally scheduled for 2001 but was scotched in the post-9/11 climate of fear. You’ll notice that, this year, the dark musical about U.S. presidential assassinations is being held after the election. Even though the threat of a kook taking a shot is always with us.