If by build tools you mean things such as CMake or Meson, then I’d argue you should punt as much as possible to them, including Python-specific tasks such as compiling bytecode etc.
So what you are left with is an elaborate frontend to orchestrate the generation of CMake or Meson configuration to build the package at hand.
And perhaps you don’t even need the frontend and this can all be done as Meson plugins written in Python (does Meson have plugins?).
(this is only for the build side, though)