Python 3.13 support

[nitzmahone]

Now that Python 3.13.0b1 is out, time to get at least a 6.0.2 pre-release out the door with 3.13 support. Most everything is working, but there are a couple of deferred housekeeping things that I'm still finishing up...

• [x] Support for Cython 3.0+ (via conditional old_build_ext import)
• [x] Custom cibuildwheel with 3.13 support
• [ ] Update CI.yaml to use dynamic matrix (eliminate extra copy for release)
• [ ] Support for macos-14 GHA runner (native Apple Silicon, reduces need for our self-hosted runners)
• [ ] Update libyaml Mac shared build to work with newer autoconf
• [ ] Support for windows-2022 GHA runner
• [ ] Update libyaml Windows shared build to run under VS2022

[Uzume]

In regard to "Support for Cython 3.0+ (via conditional old_build_ext import)", is a #731/#602 style "fix" (to get around cython/cython#4498) really a good idea? Wouldn't it be a better idea to move away from cython_sources, Cython.Distutils.build_ext/Cython.Distutils.old_build_ext and the deprecated distutils to the supported Cython.Build.build_ext? Cython likely means to drop support for those deprecated interfaces at some point (especially since python/cpython#92584). Wouldn't a #729 style fix (at build time only; see my comments there) be a better approach since Distributing Cython modules seems to be the recommended and supported method (and presumably is more unlikely to break in the future)?