input: Write a python function to calculate the mean of a numpy array.
  assert np.isclose(calculate_mean(np.array([2, 4, 7, 8, 9])), 6.2)
  assert np.isclose(calculate_mean(np.array([1, 2, 3, 4, 5])), 3.0)
  assert np.isclose(calculate_mean(np.array([10, 20, 30, 40, 50])), 30.0)
output: import numpy as np
  
  def calculate_mean(arr):
      return np.mean(arr)