bpo-35886: Make PyInterpreterState an opaque type in the public API. (GH-11731)

Move PyInterpreterState into the "internal" header files.
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 85810f3..350ef77 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -4685,7 +4685,7 @@
 get_core_config(PyObject *self, PyObject *Py_UNUSED(args))
 {
     PyInterpreterState *interp = _PyInterpreterState_Get();
-    const _PyCoreConfig *config = &interp->core_config;
+    const _PyCoreConfig *config = _PyInterpreterState_GetCoreConfig(interp);
     return _PyCoreConfig_AsDict(config);
 }
 
@@ -4694,7 +4694,7 @@
 get_main_config(PyObject *self, PyObject *Py_UNUSED(args))
 {
     PyInterpreterState *interp = _PyInterpreterState_Get();
-    const _PyMainInterpreterConfig *config = &interp->config;
+    const _PyMainInterpreterConfig *config = _PyInterpreterState_GetMainConfig(interp);
     return _PyMainInterpreterConfig_AsDict(config);
 }