test Python 3.12, drop Python 3.8 as it is EOL

[mfenner]

Description

Drop testing with Python 3.8, add testing with 3.12.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [x] I'm aware of the code of conduct.
• [x] I've created logical separate commits and followed the commit message format.
• [ ] I've added relevant test cases.
• [x] I've added relevant documentation.
• [ ] I've marked translation strings.
• [ ] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [x] I've NOT included third-party code (copy/pasted source code or new dependencies).
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[tmorrell]

Some other CI updates were needed, and I've integrated these changes into https://github.com/inveniosoftware/datacite/pull/97