input: Write a function to implement bubble sort and sort a list of integers.
  assert bubble_sort([2, 7, 4, 1, 5, 3]) == [1, 2, 3, 4, 5, 7]
  assert bubble_sort([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  assert bubble_sort([5, 1, 4, 2, 8]) == [1, 2, 4, 5, 8]
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