Add PyThreadState_SetAsyncExc(long, PyObject *).

A new API (only accessible from C) to interrupt a thread by sending it
an exception.  This is not always effective, but might help some people.
Requested by Just van Rossum and Alex Martelli.  It is intentional
that you have to write your own C extension to call it from Python.

Docs will have to wait.
diff --git a/Include/pystate.h b/Include/pystate.h
index c1182a6..63a5cb8 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -74,6 +74,9 @@
     int tick_counter;
     int gilstate_counter;
 
+    PyObject *async_exc; /* Asynchronous exception to raise */
+    long thread_id; /* Thread id where this tstate was created */
+
     /* XXX signal handlers should also be here */
 
 } PyThreadState;
@@ -93,6 +96,7 @@
 PyAPI_FUNC(PyThreadState *) PyThreadState_Get(void);
 PyAPI_FUNC(PyThreadState *) PyThreadState_Swap(PyThreadState *);
 PyAPI_FUNC(PyObject *) PyThreadState_GetDict(void);
+PyAPI_FUNC(int) PyThreadState_SetAsyncExc(long, PyObject *);
 
 
 /* Variable and macro for in-line access to current thread state */