input: Write a python function to generate a histogram dictionary from a list of numbers.
  assert histogram([1, 2, 2, 3, 3, 3]) == {1: 1, 2: 2, 3: 3}
  assert histogram([4, 4, 4, 4, 5, 5, 6]) == {4: 4, 5: 2, 6: 1}
  assert histogram([1, 1, 1, 2, 3, 3, 4, 4, 4, 4]) == {1: 3, 2: 1, 3: 2, 4: 4}
output: def histogram(numbers):
      histogram_dict = {}
      for number in numbers:
          if number in histogram_dict:
              histogram_dict[number] += 1
          else:
              histogram_dict[number] = 1
      return histogram_dict