input: Write a python function to replace vowels in a string with underscores.
  assert replace_vowels("Hello World") == "H_ll_ W_rld"
  assert replace_vowels("Python Programming") == "Pyth_n Pr_gr_mm_ng"
  assert replace_vowels("AEIOUaeiou") == "__________"
output: def replace_vowels(s: str) -> str:
      vowels = "AEIOUaeiou"
      return ''.join('_' if char in vowels else char for char in s)