input: Write a python function to manage a collection of books with ISBN and title using a class.
  assert book_manager.get_book("978-3-16-148410-0") == "The Great Gatsby"
  assert book_manager.get_book("978-0-14-044913-6") == "The Odyssey"
  assert book_manager.get_book("123-4-56-789012-3") == None
output: class BookManager:
      def __init__(self):
          self.books = {}
      
      def add_book(self, isbn, title):
          self.books[isbn] = title
      
      def get_book(self, isbn):
          return self.books.get(isbn)
  
  # Create an instance of BookManager
  book_manager = BookManager()
  
  # Add some books
  book_manager.add_book("978-3-16-148410-0", "The Great Gatsby")
  book_manager.add_book("978-0-14-044913-6", "The Odyssey")
  
  # Assertions
  assert book_manager.get_book("978-3-16-148410-0") == "The Great Gatsby"
  assert book_manager.get_book("978-0-14-044913-6") == "The Odyssey"
  assert book_manager.get_book("123-4-56-789012-3") == None