Migrate from PyPDF2 to pypdf and clean up obsolete mobi_to_json test

[PRAZPC]

Describe the bug

1. PyPDF2 is deprecated and has been superseded by pypdf.
2. The function mobi_to_json has been removed from the dev, but a corresponding test case still exists

Expected behavior
The project should use pypdf instead of the deprecated PyPDF2 library.

Additional context
I have migrated the PDF-handling code locally from PyPDF2 → pypdf.

I also identified the outdated test case related to mobi_to_json and can update it according to the maintainers’ preference.