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