input: Write a python function to create an instance of the Book class and return its string representation.
  assert create_book_string("1984", "George Orwell", 1949) == "1984 by George Orwell published in 1949."
  assert create_book_string("To Kill a Mockingbird", "Harper Lee", 1960) == "To Kill a Mockingbird by Harper Lee published in 1960."
  assert create_book_string("The Great Gatsby", "F. Scott Fitzgerald", 1925) == "The Great Gatsby by F. Scott Fitzgerald published in 1925."
output: class Book:
      def __init__(self, title, author, year):
          self.title = title
          self.author = author
          self.year = year
      
      def __str__(self):
          return f"{self.title} by {self.author} published in {self.year}."
  
  def create_book_string(title, author, year):
      book = Book(title, author, year)
      return str(book)