input: Write a python function to count the number of vowels in a given string.
  assert countVowels('Hello, World!') == 3
  assert countVowels('Python is fun') == 3
  assert countVowels('AEIOU aeiou') == 10
output: def countVowels(s):
      vowels = 'aeiouAEIOU'
      return sum(1 for char in s if char in vowels)