input: Write a function to calculate both the area and circumference of a circle given its radius.
  assert area_circumference(1) == (3.141592653589793, 6.283185307179586)
  assert area_circumference(2) == (12.566370614359172, 12.566370614359172)
  assert area_circumference(0) == (0, 0)
output: def area_circumference(radius):
      import math
      area = math.pi * radius ** 2
      circumference = 2 * math.pi * radius
      return area, circumference