gh-81057: Add PyInterpreterState.static_objects (gh-99397)

As we consolidate global variables, we find some objects that are almost suitable to add to _PyRuntimeState.global_objects, but have some small/sneaky bit of per-interpreter state (e.g. a weakref list). We're adding PyInterpreterState.static_objects so we can move such objects there. (We'll removed the _not_used field once we've added others.)

https://github.com/python/cpython/issues/81057

Include/internal/pycore_runtime_init.h

@@ -77,6 +77,11 @@ extern "C" {
                 { .threshold = 10, }, \
             }, \
         }, \
+        .static_objects = { \
+            .singletons = { \
+                ._not_used = 1, \
+            }, \
+        }, \
         ._initial_thread = _PyThreadState_INIT, \
     }