python-flit-core: revert to 3.12.0

python-flit-core: revert to 3.12.0

flit-core 4.0.2 breaks the whole python feed. flit-core is a host-only
build backend, but nearly everything built with it caps flit_core<4 (the
host tools wheel/installer/pyproject-hooks/typing-extensions and dozens of
leaf packages like marshmallow, click, pip), so their host builds fail with
"Missing dependencies: flit_core<4" against 4.0.2. Nothing needs
flit_core>=4; 3.12.0 satisfies every cap and has no runtime impact. Revert
until the ecosystem's upper bounds catch up.

Signed-off-by: Alexandru Ardelean <redacted>