Add library homework

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1. Created Library class and logic for adding and removing books, as well as iteration and generating books by author.
2. Created Publication class as an abstract base for Book, Magazine (and possibly other classes in the future).
3. Created a Pydantic model for publications.
4. Created decorators to coordinate the process of adding to and removing books from the library.
5. Implemented import/export of library content using a file.