input: Write a python function to calculate BMI (Body Mass Index) using weight and height.
  assert calculate_bmi(70, 1.75) == 22.857142857142858
  assert calculate_bmi(90, 1.9) == 24.930747922437675
  assert calculate_bmi(50, 1.5) == 22.22222222222222
output: def calculate_bmi(weight, height):
      return weight / (height ** 2)