Update pdfminer.six to v20251107 and require Python 3.9+

[pbottine]

This update addresses issue #3434 by updating the pdfminer.six dependency from version 20240706 (July 2024) to the latest version 20251107 (November 2025).

Key changes:

• Update pdfminer.six from >=20220524,<=20240706 to >=20251107
• Update python_requires from >=3.8 to >=3.9
• Remove Python 3.8 from CI test matrix in GitHub Actions

Security fix: Version 20251107 includes a critical fix for arbitrary code execution when loading pickle cmaps (CVE pending). This vulnerability could allow an attacker to execute arbitrary code by crafting a malicious PDF file with specially-crafted pickle-serialized character map data.

Breaking change: pdfminer.six dropped Python 3.8 support in v20250327, requiring this project to also bump its minimum Python version to 3.9.

Testing: All unit tests pass, and PDF parsing functionality has been verified with sample PDF files.

Fixes #3434

cc: @smoelius

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