bpo-39947: Add PyThreadState_GetInterpreter() (GH-18981)

Add PyThreadState_GetInterpreter(tstate): get the interpreter of a
Python thread state.
diff --git a/Include/pystate.h b/Include/pystate.h
index 57dc9e4..70d3bdc 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -87,6 +87,11 @@
 PyAPI_FUNC(PyObject *) PyThreadState_GetDict(void);
 PyAPI_FUNC(int) PyThreadState_SetAsyncExc(unsigned long, PyObject *);
 
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03090000
+/* New in 3.9 */
+PyAPI_FUNC(PyInterpreterState *) PyThreadState_GetInterpreter(PyThreadState *tstate);
+#endif
+
 typedef
     enum {PyGILState_LOCKED, PyGILState_UNLOCKED}
         PyGILState_STATE;