clear the thread local PThreadState on shutdown

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/PythonContext.java

@@ -694,6 +694,10 @@ public void runShutdownHooks() {
         for (ShutdownHook h : shutdownHooks) {
             h.call(this);
         }
+        // destroy thread state
+        if (customThreadState != null) {
+            customThreadState.set(null);
+        }
     }
 
     @TruffleBoundary