Migrate from PyPDF2 to pypdf and remove obsolete mobi_to_json test

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Pull Request Template

What have you Changed

• Migrated the project from PyPDF2 to pypdf, replacing deprecated imports and updating PDF-handling code accordingly.
  Updated the code to use the modern PdfReader API.
• Removed/cleaned up the obsolete mobi_to_json test case, which remained in the test suite even though the function was removed from the codebase.
• Verified that all tests pass successfully after the migration and cleanup.

Issue no.(must) - #87

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• One Change in one Pull Request
• I am following clean code and Documentation and my code is well linted with flake8.

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Thanks for improving the audiobook.

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Pull request overview

This PR modernizes the PDF handling library by migrating from the deprecated PyPDF2 to its actively maintained successor pypdf. The migration updates the dependency, refactors all PDF-related code to use the new API, and cleans up an obsolete test case for removed mobi functionality.

Key changes:

• Updated dependency from PyPDF2 3.0.1 to pypdf 4.0.1 with corresponding API migrations (PdfFileReader → PdfReader, method name updates)
• Renamed PyPDF2DocParser class to PyPDFDocParser to reflect the new library name
• Removed obsolete mobi_to_json test case that referenced a function no longer in the codebase

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
requirements.txt	Updated PDF library dependency from PyPDF2 3.0.1 to pypdf 4.0.1
audiobook/doc_parser/pdf_parser.py	Migrated to pypdf API: updated imports, class name, and all method calls (PdfFileReader→PdfReader, numPages→len(pages), getPage→pages[], extractText→extract_text, getOutlines→outline)
audiobook/utils.py	Updated import statement to use PyPDFDocParser instead of PyPDF2DocParser
audiobook/main.py	Updated logger name from "PyPDF2" to "pypdf" to align with new library
tests/test_create_json_book.py	Renamed test from test_pdf_to_json_pypdf2 to test_pdf_to_json_pypdf; commented out obsolete mobi_to_json test
docs/command_line_usage.rst	Updated documentation to reference pypdf instead of pypdf2 in extraction engine table

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This comment appears to contain commented-out code.

Suggested change

	# def test_docs_to_json(self):
	# self.assertEqual(ab.create_json_book("assets/sample.doc"), (output['docs'], {'book_name': 'sample', 'pages': 1}))
	@unittest.skip("DOC to JSON test is currently disabled (e.g., due to missing support or failing test).")
	def test_docs_to_json(self):
	self.assertEqual(ab.create_json_book("assets/sample.doc"), (output['docs'], {'book_name': 'sample', 'pages': 1}))