Description
For historical reasons pip uses setuptools to build pip's distribution files, however other build back ends are now possible.
I propose switching to flit-core, for the main reason being stability, the last major release was in 2020, whereas setuptools has had 5 major releases this year so far. flit-core explicitly recommends pinning to it's current major version.
flit-core is a fellow PyPA project and is maintained in https://github.com/pypa/flit. No vendoring is required as this all done during build, there should be no run time changes. Though informing the flit project would probably be a nice courtesy.
One consideration is tools and organizations that build pip from source, this may affect them if they have hard coded that pip uses setuptools to build. However such hard coding has not been required for a long time, and flit has been around for 10 years now, so has plenty of time to be vendored by large projects (such as linux distros).