input: Write a python function to create a Book object and return its string representation.
  assert str(create_book('1984', 'George Orwell')) == '1984 by George Orwell'
  assert str(create_book('To Kill a Mockingbird', 'Harper Lee')) == 'To Kill a Mockingbird by Harper Lee'
  assert str(create_book('The Great Gatsby', 'F. Scott Fitzgerald')) == 'The Great Gatsby by F. Scott Fitzgerald'
output: def create_book(title, author):
      class Book:
          def __init__(self, title, author):
              self.title = title
              self.author = author
  
          def __str__(self):
              return f"{self.title} by {self.author}"
  
      return Book(title, author)