input: Write a python function to replace vowels in a string with a specified character.
  assert replace_vowels("replace all vowels", "*") == "r*pl*c* *ll v*w*ls"
  assert replace_vowels("hello world", "#") == "h#ll# w#rld"
  assert replace_vowels("testing vowels", "!") == "t!st!ng v!w!ls"
output: def replace_vowels(s, ch):
      vowels = "aeiouAEIOU"
      return ''.join([ch if char in vowels else char for char in s])