Add C API equivalent for `sys._is_gil_enabled()`

[devinrsmith]

Feature or enhancement

Proposal:

sys._is_gil_enabled is documented as a CPython implementation detail - it would be good to have a C API to expose this in a stable way to C modules. Possibly, something like

int PyInterpreterState_IsGilEnabled(PyInterpreterState *interp)

We'd like to have a stable way to expose this information downstream, jpy-consortium/jpy#218

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response