Upgrade to Python 3.14 #6702

Summary

• Upgrade CI to 3.14
• Mark version 3.14
• Make basic library works
  • site
    • upgrade test_site again once unittest is upgraded
  • platform
    • upgrade test_platform again once unittest is upgraded
  • ensurepip
  • venvlauncher
• Make sure CI passes

not included

• annotationlib
• inspect
• enum
• dataclasses

Details