input: Write a function to perform a valid insertion sort on a given list of integers.
  def insertion_sort(arr):
   for i in range(1, len(arr)):
    key = arr[i]
    j = i -1
    while j >= 0 and key < arr[j] :
      arr[j + 1] = arr[j]
      j = j - 1
    arr[j + 1] = key
   return arr
  assert insertion_sort([4,3,2,10,12,1,5,6])==[1,2,3,4,5,6,10,12]
  assert insertion_sort([1,2,3,4,5,6,7])==[1,2,3,4,5,6,7]
  assert insertion_sort([6,5,4,3,2,1])==[1,2,3,4,5,6]
  assert insertion_sort([0,0,0,0,0,0])==[0,0,0,0,0,0]
  assert insertion_sort([])==[]
output: def insertion_sort(arr):
   for i in range(1, len(arr)):
    key = arr[i]
    j = i -1
    while j >= 0 and key < arr[j] :
      arr[j + 1] = arr[j]
      j = j - 1
    arr[j + 1] = key
   return arr