input: Write a python function to calculate the mean of a list of integers and return a string of the mean value.
  assert calculate_mean([1, 2, 3, 4, 5]) == 'The mean is 3.0'
  assert calculate_mean([10, 20, 30, 40, 50]) == 'The mean is 30.0'
  assert calculate_mean([7, 8, 9]) == 'The mean is 8.0'
output: def calculate_mean(numbers):
      mean_value = sum(numbers) / len(numbers)
      return f'The mean is {mean_value}'