input: Write a python function to count the number of vowels in a string.
  assert count_vowels("hello world") == 3
  assert count_vowels("AEIOUaeiou") == 10
  assert count_vowels("bcdfghjklmnpqrstvwxyz") == 0
output: def count_vowels(s):
      vowels = "AEIOUaeiou"
      return sum(1 for char in s if char in vowels)