input: Write a python function to replace vowels in a string with underscores.
  assert replace_vowels("hello")== "h_ll_"
  assert replace_vowels("world")== "w_rld"
  assert replace_vowels("AEIOU")== "_____"
  assert replace_vowels("Python")== "Pyth_n"
output: def replace_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join('_' if char in vowels else char for char in s)