input: Write a python function to count the number of vowels in a given string.
  assert count_vowels('This is a test string for counting vowels.') == 10
  assert count_vowels('Hello World') == 3
  assert count_vowels('Python Programming') == 4
  assert count_vowels('abcdefghijklmnopqrstuvwxyz') == 5
output: def count_vowels(s):
      vowels = 'aeiouAEIOU'
      return sum(1 for char in s if char in vowels)