[memegauste]

The Python 3.14 CI job fails while building autopep8 1.5.4, which reads the removed ast.Constant.s attribute. Test requirements also install the entire development toolchain, and the pinned Django and test tools predate the newer Python versions in the matrix.

• Upgrade to Django 5.2.18 LTS and compatible runtime, pytest, coverage, and lint dependencies; keep setup.py requirements aligned.
• Separate test/lint requirements from optional development tools, and update autopep8, IPython, and Sphinx.
• Upgrade checkout/setup-python to v7, cache pip dependencies, check dependency consistency, split lint into its own step, and let every Python 3.12–3.15 matrix job finish.
• Replace the tests' cgi dependency with Django response.charset and make lint compatibility adjustments while preserving payment behavior.

Validation: all 29 tests and flake8 pass locally on Python 3.14.8 and 3.15.0; pip check, development dependency resolution, wheel build, and git diff --check pass. The pull request runs the full Linux matrix.

Django 5.2 officially documents support through Python 3.14; Python 3.15 remains an additional project compatibility check.