input: Write a function to replace all vowels in a string with the letter 'X'.
  assert replace_vowels("Hello, world!") == "HXllX, wXrld!"
  assert replace_vowels("aeiou") == "XXXXX"
  assert replace_vowels("This is a test.") == "ThXs Xs X tXst."
output: def replace_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join('X' if char in vowels else char for char in s)