input: Write a python function to count the number of vowels in a given string.
  assert count_vowels("Hello World") == 3
  assert count_vowels("Python Programming") == 4
  assert count_vowels("AEIOU") == 5
output: def count_vowels(s):
      vowels = set("aeiouAEIOU")
      return sum(1 for char in s if char in vowels)