Move to the back

A customer at the front of the line forgot their wallet and goes to the back. Write to_back(line) that takes a deque, moves the front item to the back, and returns the deque you were given, not a new one. An empty deque is returned unchanged.

to_back(deque(["Ada", "Grace", "Linus"]))  ->  deque(["Grace", "Linus", "Ada"])

from collections import deque

def to_back(line):
    ...