input: Write a python function to perform bubble sort on a list of integers.
  assert bubble_sort([3, 5, 4, 1, 6]) == [1, 3, 4, 5, 6]
  assert bubble_sort([10, 2, 33, 21, 5]) == [2, 5, 10, 21, 33]
  assert bubble_sort([3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]) == [1, 1, 2, 3, 3, 4, 5, 5, 5, 6, 9]
output: def bubble_sort(arr):
      n = len(arr)
      for i in range(n):
          for j in range(0, n-i-1):
              if arr[j] > arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
      return arr