input: Write a python function to extract vowels from a given string.
  assert extract_vowels("Hello World!") == "eoo"
  assert extract_vowels("Python Programming") == "o oai"
  assert extract_vowels("Data Science") == "a aeie"
output: def extract_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char in vowels])