Do not use legacy system

[henri-gasc]

While installing the package, we get the following error:

INFO Building wheel via backend setuptools.build_meta:__legacy__
<string>:18: SetuptoolsDeprecationWarning: The test command is disabled and references to it are deprecated.
!!
        ********************************************************************************
        Please remove any references to `setuptools.command.test` in all supported versions of the affected package.

        This deprecation is overdue, please update your project and remove deprecated
        calls to avoid build errors in the future.
        ********************************************************************************
!!
/usr/lib/pypy3.10/site-packages/setuptools/_distutils/dist.py:270: UserWarning: Unknown distribution option: 'tests_require'
  warnings.warn(msg)

(There is another concerning bdist_wheel.universal but it is already removed in the live version so it is not a problem).
It is not clear when this will stop working, but it would be great to make it future-proof.

From what I understand, it would also be better to use pyproject.toml rather than setup.cfg to avoid using setuptools.build_meta:__legacy__