SERIES RETURN: Ten years ago, when this series was launched, it was copying a formula that had worked in England, but which had no guarantee of being transplanted successfully to the United States. Ballroom dancing in prime time? Really? Well, a decade later, Dancing with the Stars begins competition number 20, and its roster, this time, includes two attention-getting divas from the Seventies: singer Patti LaBelle and Three’s Company star Suzanne Somers, who was part of the mix the first time ABC ruled the ratings, also way back in the Seventies. Tom Bergeron hosts.