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 t