input: Write a python function to find the last occurrence of an item in a list.
  assert last_occurrence([1, 2, 3, 4, 2, 5], 2) == 4
  assert last_occurrence([1, 2, 3, 4, 5], 6) == -1
  assert last_occurrence([5, 5, 5, 5, 5], 5) == 4
  assert last_occurrence([], 1) == -1
output: def last_occurrence(lst, item):
      for i in range(len(lst) - 1, -1, -1):
          if lst[i] == item:
              return i
      return -1