input: Write a python function to find the middle element of a list.
  assert middle_element([1, 3, 5, 7, 9, 11]) == 7
  assert middle_element([2, 4, 6, 8, 10]) == 6
  assert middle_element([10, 20, 30, 40, 50, 60, 70]) == 40
output: def middle_element(lst):
      mid_index = len(lst) // 2
      return lst[mid_index]