gh-76785: Handle Legacy Interpreters Properly (gh-117490)

This is similar to the situation with threading._DummyThread.  The methods (incl. __del__()) of interpreters.Interpreter objects must be careful with interpreters not created by interpreters.create().  The simplest thing to start with is to disable any method that modifies or runs in the interpreter.  As part of this, the runtime keeps track of where an interpreter was created.  We also handle interpreter "refcounts" properly.

Python/pystate.c

@@ -1111,6 +1111,13 @@ _PyInterpreterState_ReinitRunningMain(PyThreadState *tstate)
 // accessors
 //----------
 
+int
+_PyInterpreterState_IsReady(PyInterpreterState *interp)
+{
+    return interp->_ready;
+}
+
+
 static inline int
 check_interpreter_whence(long whence)
 {