input: Write a python function to encrypt a string by shifting vowels to the next vowel in the sequence.
  assert encrypt_string("hello") == "hillu"
  assert encrypt_string("python") == "pythun"
  assert encrypt_string("programming") == "prugremmong"
  assert encrypt_string("encrypt") == "encrupt"
output: def encrypt_string(s):
      vowels = 'aeiou'
      next_vowel = {vowels[i]: vowels[(i + 1) % len(vowels)] for i in range(len(vowels))}
      
      result = []
      for char in s:
          if char in vowels:
              result.append(next_vowel[char])
          else:
              result.append(char)
      
      return ''.join(result)