input: Write a function to perform bubble sort on a list.
  assert bubble_sort([3, 2, 5, 6, 1, 4])==[1, 2, 3, 4, 5, 6]
  assert bubble_sort([10, 7, 8, 9, 1, 5])==[1, 5, 7, 8, 9, 10]
  assert bubble_sort([20, 15, 25, 30, 10])==[10, 15, 20, 25, 30]
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