input: Write a function to sum all elements in a list that are divisible by 3.
  def divisibleBy3(li):
   total = 0
   for num in li:
    if num % 3 == 0:
     total += num
   return total
  assert divisibleBy3([4, 6, 17, 24, 5, 9])==24
output: def divisibleBy3(li):
   total = 0
   for num in li:
    if num % 3 == 0:
     total += num
   return total