feat: add Python 3.12 support and drop Python 3.8

[naincy128]

Upgrade to Python 3.12

Changes

File	Change	Why
helpers.py	from marshmallow import RAISE	marshmallow 4 removed RAISE from marshmallow.utils
setup.py	marshmallow<4	marshmallow 4 also removes APIs helpers.Discriminator uses; porting is separate work
setup.py	python_requires>=3.11, classifiers 3.11/3.12	New supported versions
tox.ini, python-test.yml	Test on 3.11 and 3.12	New supported versions
python-release.yml, Makefile	Release on 3.12, install setuptools	Python 3.12 no longer includes setuptools
python-test.yml	include-hidden-files: true on coverage upload	Hidden .coverage.* files were skipped, so the coverage job failed
test_client.py	sleep(1) → sleep(2) in test_auto_refresh	Fixes an intermittent timing failure
setup.py, version.py, CHANGES.md	Version 2026.9.28	Release

Testing

• tox -e py311,py312: 173 passed on each
• tox -e format and tox -e coverage-report: OK