input: Write a function to count the number of vowels in a given string. If there are no vowels in the string, return 0.
  def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count
  assert count_vowels("Hello World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("Stranger Things")==3
  assert count_vowels("rhythm")==0
  assert count_vowels("")==0
output: def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count