6.0.2 final release checklist for Python 3.13 support

[nitzmahone]

Now that Python 3.13.0rc1 has been tagged, it's time to do a final release of 6.0.2 with 3.13 support.

• Fix or kill setup.py test (YAML 1.2 Core Schema support (about yes/no, on/off, 07 vs 08) #486
• Update python_requires metadata to exclude < 3.8
• Update CI to test against 3.13.0rc1 (once manylinux and cibuildwheel are updated with 3.13.0rc1 support, or use private copies)
• Do 6.0.2 final build and release
• Yank 6.0.2rc1 from PyPI so < 3.8 users installing with --pre don't pick up something that won't build (but it's still available for anyone explicitly installing pyyaml==6.0.2rc1