input: Write a function to calculate the mean of an array of integers using numpy.
  assert get_mean([1, 2, 3, 4, 5]) == 3.0
  assert get_mean([10, 20, 30, 40, 50]) == 30.0
  assert get_mean([100, 200, 300, 400, 500]) == 300.0
output: import numpy as np
  
  def get_mean(arr):
      return np.mean(arr)