CI: lint fails with current flake8; matrix tests end-of-life Python versions #326

Two problems in the Python package workflow on master (91e334d):

1. The Lint with flake8 step fails with current flake8. flake8 7.2.0 and 7.4.1 report F824 for the two unused global coreparams statements in bitcoin/core/__init__.py (lines 30 and 773); 7.1.2 does not. The workflow installs flake8 unpinned, so it picks up 7.4.1 today.

2. The matrix tests Python 3.9, 3.10 and 3.11. 3.9 reached end-of-life on 2025-10-31 and 3.10 on 2026-10-01; 3.12 to 3.14 are not tested. The workflow also uses actions/setup-python@v3 and actions/checkout@v4; the current major version of both is v7.