input: Write a python function to count the number of books in the BookDB.
  assert count_books(db) == 0  # db is an empty BookDB instance
  db.add_book(Book("1984", "George Orwell", 1949))
  assert count_books(db) == 1
  db.add_book(Book("To Kill a Mockingbird", "Harper Lee", 1960))
  assert count_books(db) == 2
  db.delete_book("1984")
  assert count_books(db) == 1
output: def count_books(db):
      return len(db.books)