input: Develop a function named 'count_vowels' that accepts a string as input and returns how many vowels it contains. For this context, 'a', 'e', 'i', 'o', 'u' are considered vowels, and 'y' will also be treated as a vowel if it appears at the end of the string.
  assert count_vowels("abcde") == 2
  assert count_vowels("Alone") == 3
  assert count_vowels("key") == 2
  assert count_vowels("bye") == 1
  assert count_vowels("keY") == 2
  assert count_vowels("bYe") == 1
  assert count_vowels("ACEDY") == 3
output:     
      vowels = 'aeiouAEIOU'
      count = sum(1 for char in s if char in vowels)
      if s and s[-1] in 'yY':
          count += 1
      return count
  