This PR adds the ability to enable the GIL if it was disabled at
interpreter startup, and modifies the multi-phase module initialization
path to enable the GIL when loading a module, unless that module's spec
includes a slot indicating it can run safely without the GIL.
PEP 703 called the constant for the slot `Py_mod_gil_not_used`; I went
with `Py_MOD_GIL_NOT_USED` for consistency with gh-104148.
A warning will be issued up to once per interpreter for the first
GIL-using module that is loaded. If `-v` is given, a shorter message
will be printed to stderr every time a GIL-using module is loaded
(including the first one that issues a warning).
Argument Clinic now has a partial support of the
Limited API:
* Add --limited option to clinic.c.
* Add '_testclinic_limited' extension which is built with
the limited C API version 3.13.
* For now, hardcode in clinic.py that "_testclinic_limited.c" targets
the limited C API.
