input: Write a python function to perform bubble sort on a list of integers.
  assert bubble_sort([4, 2, 9, 12, 3, 5, 7]) == [2, 3, 4, 5, 7, 9, 12]
  assert bubble_sort([5, 1, 4, 2, 8]) == [1, 2, 4, 5, 8]
  assert bubble_sort([20, 10, 30, 25, 15]) == [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