input: Write a python function to replace all vowels in a string with underscores.
  assert replace_vowels('teststring') == 't_ststr_ng'
  assert replace_vowels('HELLO') == 'H_LL_'
  assert replace_vowels('Python') == 'Pyth_n'
output: def replace_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join('_' if c in vowels else c for c in s)