gh-101659: Isolate "obmalloc" State to Each Interpreter (gh-101660)

This is strictly about moving the "obmalloc" runtime state from
`_PyRuntimeState` to `PyInterpreterState`.  Doing so improves isolation
between interpreters, specifically most of the memory (incl. objects)
allocated for each interpreter's use.  This is important for a
per-interpreter GIL, but such isolation is valuable even without it.

FWIW, a per-interpreter obmalloc is the proverbial
canary-in-the-coalmine when it comes to the isolation of objects between
interpreters.  Any object that leaks (unintentionally) to another
interpreter is highly likely to cause a crash (on debug builds at
least).  That's a useful thing to know, relative to interpreter
isolation.

Include/internal/pycore_pystate.h

@@ -33,6 +33,13 @@ _Py_IsMainInterpreter(PyInterpreterState *interp)
     return (interp == _PyInterpreterState_Main());
 }
 
+static inline int
+_Py_IsMainInterpreterFinalizing(PyInterpreterState *interp)
+{
+    return (_PyRuntimeState_GetFinalizing(interp->runtime) != NULL &&
+            interp == &interp->runtime->_main_interpreter);
+}
+
 
 static inline const PyConfig *
 _Py_GetMainConfig(void)