input: Write a function to sort a list of numbers using the bubble sort algorithm.
  assert bubble_sort([3, 5, 1, 4, 7]) == [1, 3, 4, 5, 7]
  assert bubble_sort([10, 2, 8, 6, 7, 3]) == [2, 3, 6, 7, 8, 10]
  assert bubble_sort([1, 2, 3, 4, 5]) == [1, 2, 3, 4, 5]
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