Add support for Python 3.13, drop EOL 3.8

[hugovk]

• Add support for Python 3.13: https://discuss.python.org/t/python-3-13-0-final-has-been-released/66972/1
• Drop support for EOL Python 3.8: https://discuss.python.org/t/python-3-8-is-now-officially-eol/66983/1
• Upgrade syntax with pyupgrade --py39-plus
• Remove universal wheels, only needed when supporting both Python 2 and 3
• Remove redundant version checks
• Fix tox: 'failed with make is not allowed, use allowlist_externals to allow it'

[nicholasserra]

Thanks! Love these, they keep me up to date on things personally.