Answer:
def sillystring(input):
output = ""
vowels = ['a', 'e', 'i', 'o', 'u']
for letter in input:
if (letter.lower() in vowels):
output += "oob"
else:
output += letter
return output
Testing:
>>> print(sillystring('Hello, world!'))
Hooblloob, woobrld!