cloudpickle doesn't build anymore from sources with flit_core>=4

To reproduce : 
`pip install --no-binary cloudpickle cloudpickle`
brings the error : 
```
Collecting cloudpickle
  Using cached cloudpickle-3.1.2.tar.gz (22 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error

  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [18 lines of output]
      Traceback (most recent call last):
        File "/home/baptiste/mfext/src/jbv/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
          main()
          ~~~~^^
        File "/home/baptiste/mfext/src/jbv/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 373, in main
          json_out["return_val"] = hook(**hook_input["kwargs"])
                                   ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
        File "/home/baptiste/mfext/src/jbv/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 143, in get_requires_for_build_wheel
          return hook(config_settings)
        File "/tmp/pip-build-env-bb5_lk9_/overlay/lib/python3.13/site-packages/flit_core/buildapi.py", line 22, in get_requires_for_build_wheel
          info = read_flit_config(pyproj_toml)
        File "/tmp/pip-build-env-bb5_lk9_/overlay/lib/python3.13/site-packages/flit_core/config.py", line 75, in read_flit_config
          return prep_toml_config(d, path)
        File "/tmp/pip-build-env-bb5_lk9_/overlay/lib/python3.13/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.
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
ERROR: Failed to build 'cloudpickle' when getting requirements to build wheel

```

In `pyproject.toml` the `[tool.flit.metadata]` section should be replaced by a standard `[project]` section or you should replace `requires = ["flit_core"]` by `requires = ["flit_core <4"]` in the `[build-system]` section

Regards

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