input: Write a python function to count the number of vowels in a given string.
  assert vowel_count("Hello World") == 3
  assert vowel_count("Python Programming") == 4
  assert vowel_count("AI is fascinating") == 7
output: def vowel_count(s):
      vowels = 'aeiouAEIOU'
      count = sum(1 for char in s if char in vowels)
      return count