input: Write a function to count the number of vowels in a given string.
  assert count_vowels("hello") == 2
  assert count_vowels("world") == 1
  assert count_vowels("programming") == 3
output: def count_vowels(s):
      vowels = 'aeiouAEIOU'
      return sum(1 for char in s if char in vowels)