input: from typing import List
  
  def count_vowels(string: str) -> int:
      """ Takes a string and returns the number of vowels in it.
      >>> count_vowels("hello world")
      3
      >>> count_vowels("AEIOU")
      5
      >>> count_vowels("xyz")
      0
      """
output:     vowels = "aeiouAEIOU"
      count = 0
      for char in string:
          if char in vowels:
              count += 1
      return count