Fix for flit_core 4

Please fix this so it builds with flit_core 4. Currently trying to build with flit_core gives:
```
/usr/pkg/lib/python3.14/site-packages/flit/buildapi.py:2: UserWarning: A package has specified `build-backend = "flit.buildapi"` and is being built with Flit >= 3.10. This is likely to break in a future version. Please change the backend to flit_core.buildapi, and/or specify a maximum version of Flit.
warn('A package has specified `build-backend = "flit.buildapi"` and is being '
Traceback (most recent call last):
File "/usr/pkg/lib/python3.14/site-packages/pyproject_hooks/_in_process/_in_process.py", line 389, in
main()
~~~~^^
File "/usr/pkg/lib/python3.14/site-packages/pyproject_hooks/_in_process/_in_process.py", line 373, in main
json_out["return_val"] = hook(**hook_input["kwargs"])
~~~~^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/pkg/lib/python3.14/site-packages/pyproject_hooks/_in_process/_in_process.py", line 280, in build_wheel
return _build_backend().build_wheel(
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
wheel_directory, config_settings, metadata_directory
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/usr/pkg/lib/python3.14/site-packages/flit_core/buildapi.py", line 74, in build_wheel
info = make_wheel_in(pyproj_toml, Path(wheel_directory))
File "/usr/pkg/lib/python3.14/site-packages/flit_core/wheel.py", line 222, in make_wheel_in
wb = WheelBuilder.from_ini_path(ini_path, fp)
File "/usr/pkg/lib/python3.14/site-packages/flit_core/wheel.py", line 86, in from_ini_path
ini_info = read_flit_config(ini_path)
File "/usr/pkg/lib/python3.14/site-packages/flit_core/config.py", line 75, in read_flit_config
return prep_toml_config(d, path)
File "/usr/pkg/lib/python3.14/site-packages/flit_core/config.py", line 91, in prep_toml_config
raise ConfigError(
...<3 lines>...
)
flit_core.config.ConfigError: The [tool.flit.metadata] table is no longer supported. Switch to the standard [project] table or require flit_core<4 to build this package.

TIP pass --env-dir and --sdist-extract-dir to keep the build environment and sources, then see https://build.pypa.io/en/stable/how-to/troubleshooting.html#debug-a-failed-build for help debugging a failed build
ERROR Backend subprocess exited when trying to invoke build_wheel
*** Error code 1

```
Thanks!