input: Write a python function to retrieve all books authored by a specific user.
  assert get_books_by_author(User.objects.get(id=1)) == ['Book1', 'Book2']
  assert get_books_by_author(User.objects.get(id=2)) == ['Book3']
  assert get_books_by_author(User.objects.get(id=3)) == []
output: def get_books_by_author(user):
      return [book.title for book in user.books.all()]