input: 
  def encrypt(message: str, key: str) -> str:
      """ Encrypt a message using a key by adding the ASCII value of
      the first character of the key to each character of the message.
      If the key is shorter than the message, repeat the key to cover
      the entire message. Do not use any library functions for encryption.
      
      >>> encrypt("Hello world!", "secret")
      'TfIIx≤±§MId§'
      """
  
output:     encrypted_message = []
      key_length = len(key)
      for i, char in enumerate(message):
          key_char = key[i % key_length]
          encrypted_char = chr(ord(char) + ord(key_char))
          encrypted_message.append(encrypted_char)
      return ''.join(encrypted_message)