Version 1.4.0 and roadmap

[henryiii]

I'd like to put down my plan for the next release and the changes coming soon after it.

I'd like to release 1.4.0 very soon. This will have CPython 3.12 support, drops CPython 3.6 (!), will upgrade to the latest Boost release, have a few fixes, manylinux2014+ required, and still use the classic setuptools build.

After that, I plan to drop CPython 3.7 and move to using scikit-build-core and CMake. The current build system is painful to develop with; I find myself actually developing with the alternative CMake buildsystem + PYTHONPATH anyway to avoid Setuptools caching. If we need to bump the C++ version to 17+ to make work easier for new features like sparse storage, I'd probably be okay with that. We'd also bump our macOS min version to 10.12-10.14 or so if that happens.