Sienna Spiro on Writing Candidly as a Young Woman, While Being Frank About Adoring Sinatra and Twiggy: ‘Being 20 Is Just Tricky, Period’

Sienna Spiro was something of an established artist even before her debut full-length album, “Visitor,” came out on July 3. The 20-year-old Londoner with the big ’60s hair and an even bigger, more timeless voice is officially a freshman — she will almost certainly be up for a best new artist Grammy in a few…