Drop Python-3.8 support on 2024-10

[shimizukawa]

EOL

Python 3.8: Oct 2024 https://endoflife.date/python

• update old syntax compat code
• update tox and github CI
• update pyproject.yml
• update document
• bump version X.y.z for imcompatible update

[shimizukawa]

done by #163