input: Write a python function to calculate the volume of a sphere given its radius.
  assert abs(sphere_volume(3) - 113.04) < 0.01
  assert abs(sphere_volume(5) - 523.33) < 0.01
  assert abs(sphere_volume(1) - 4.19) < 0.01
output: def sphere_volume(radius):
      import math
      volume = (4/3) * math.pi * (radius ** 3)
      return round(volume, 2)