* ENH: Globally enable Cython free-threading directive

This is the Cython equivalent of adding a `Py_mod_gil` slot with
`Py_MOD_GIL_NOT_USED` like we did in #59135.

* Use add_project_arguments

* Mark json with Py_MOD_GIL_NOT_USED & remove PYTHON_GIL env var from ci test job