Close #19787: PyThread_set_key_value() now always set the value. In Python 3.3,

the function did nothing if the key already exists (if the current value is a
non-NULL pointer).

_testcapi.run_in_subinterp() now correctly sets the new Python thread state of
the current thread when a subinterpreter is created.

Modules/_testcapimodule.c

@@ -2511,6 +2511,10 @@ run_in_subinterp(PyObject *self, PyObject *args)
     r = PyRun_SimpleString(code);
     Py_EndInterpreter(substate);
 
+    /* restore previous thread safe. It was replaced by Py_NewInterpreter()
+       which creates a new thread state. */
+    _PyThreadState_Init(mainstate);
+
     PyThreadState_Swap(mainstate);
 
     return PyLong_FromLong(r);