fix stable abi build for >3.12

[alecjacobson]

Fixes #262

See conversation I had with myself at wjakob/nanobind#1047

[alecjacobson]

Boo. This isn't working on 3.13 yet. but https://github.com/libigl/libigl-python-bindings/tree/alecjacobson/support-cp313 was.

Not sure what the important difference is.

[alecjacobson]

https://github.com/libigl/libigl-python-bindings/actions/runs/15051583334 312-abi whl works for python3.13 (at c7a138d )

source venv313/bin/activate && python -m pip install --force libigl-2.5.4.dev0-cp312-abi3-macosx_11_0_arm64.whl && python -c "import igl"

but https://github.com/libigl/libigl-python-bindings/actions/runs/15052187598 312-abi whl does not

source venv313/bin/activate && python -m pip install --force libigl-2.6.1-cp312-abi3-macosx_11_0_arm64.whl && python -c "import igl"

there's not a lot of change between these except bumping the nanobind version (hopefully that's not the issue).

[alecjacobson]

Closing this in favor of #268 which appears to be working.