input: Write a python function to replace all vowels in a string with the character '#'.
  assert replace_vowels("Hello, I am a string.") == "H#ll#, # #m # str#ng."
  assert replace_vowels("Python Programming") == "Pyth#n Pr#gr#mm#ng"
  assert replace_vowels("AEIOUaeiou") == "##########"
  assert replace_vowels("bcdfghjklmnpqrstvwxyz") == "bcdfghjklmnpqrstvwxyz"
output: def replace_vowels(s):
      vowels = "AEIOUaeiou"
      return ''.join('#' if char in vowels else char for char in s)