input: Write a function to convert all vowels in a string to lowercase.
  assert lower_vowels("HELLO World")== "hELLO wOrld"
  assert lower_vowels("Python Programming")== "PythOn PrOgrAmmIng"
  assert lower_vowels("AEIOU aeiou")== "aeiou aeiou"
output: def lower_vowels(s):
      vowels = 'AEIOU'
      result = ''
      for char in s:
          if char in vowels:
              result += char.lower()
          else:
              result += char
      return result