“Stranger Things” fans won’t have to leave the Upside Down any time soon. A live-action spinoff series and a “Stranger Things” stage play are both in development, Netflix has announced. Few details about either project were included in the announcement, which says the spinoff show will be “based on an original idea” by franchise creators Ross and Matt Duffer. The stage play, too, will be set “within the world and mythology” of “Stranger Things,” Netflix said. Netflix debuted the first seven episodes of “Stranger Things” Season 4 in May, and released the season’s final two episodes at the start…