input: Write a python function to add a book, update a book, and delete a book from a list of Book objects.
  assert len(books) == 0
  add_book('Title1', 'Author1', 123)
  assert len(books) == 1
  assert books[0].title == 'Title1'
  assert books[0].author == 'Author1'
  assert books[0].isbn == 123
  
  update_book(123, 'NewTitle1', 'NewAuthor1')
  assert books[0].title == 'NewTitle1'
  assert books[0].author == 'NewAuthor1'
  
  delete_book(123)
  assert len(books) == 0
output: class Book:
      def __init__(self, title, author, isbn):
          self.title = title
          self.author = author
          self.isbn = isbn
  
  books = []
  
  def add_book(title, author, isbn):
      new_book = Book(title, author, isbn)
      books.append(new_book)
  
  def update_book(isbn, new_title, new_author):
      for book in books:
          if book.isbn == isbn:
              book.title = new_title
              book.author = new_author
              break
  
  def delete_book(isbn):
      global books
      books = [book for book in books if book.isbn != isbn]