Currently basic initialization of PyInterprterState happens in PyInterpreterState_New() (along with allocation and adding the new interpreter to the runtime state). This prevents us from initializing interpreter states that were allocated separately (e.g. statically or in a free list). We address that here by factoring out a separate function just for initialization. We do the same for PyThreadState. _PyRuntimeState was sorted out when we added it since _PyRuntime is statically allocated. However, in this PR we update the existing init code to line up with the functions for PyInterpreterState and PyThreadState.

(There should be zero change in behavior.)

https://bugs.python.org/issue46008