input: Write a function to replace vowels in a string with '$'.
  assert replace_vowels('hello') == 'h$ll$'
  assert replace_vowels('aeiou') == '$$$$$'
  assert replace_vowels('Python') == 'Pyth$n'
  assert replace_vowels('') == ''
  assert replace_vowels('bcdfg') == 'bcdfg'
output: def replace_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join('$' if char in vowels else char for char in s)