input: Write a function that accepts a string as input and returns the total number of vowels in the string.
  assert count_vowels("hello world") == 3
  assert count_vowels("PYTHON") == 1
  assert count_vowels("aeiouAEIOU") == 10
  assert count_vowels("bcdfghjklmnpqrstvwxyz") == 0
  assert count_vowels("") == 0
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count