input: Write a function in Python to count the number of vowels in a string.
  
  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 Language")==8
  assert count_vowels("AEIOUaeiou")==10
  assert count_vowels("")==0
  assert count_vowels("Cymbals are noisy")==4
output: def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count