bpo-41435: Add sys._current_exceptions() function (GH-21689)

This adds a new function named sys._current_exceptions() which is equivalent ot
sys._current_frames() except that it returns the exceptions currently handled
by other threads. It is equivalent to calling sys.exc_info() for each running
thread.
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 5d5e4e3..25522b4 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -167,6 +167,11 @@ PyAPI_FUNC(PyInterpreterState *) _PyGILState_GetInterpreterStateUnsafe(void);
 */
 PyAPI_FUNC(PyObject *) _PyThread_CurrentFrames(void);
 
+/* The implementation of sys._current_exceptions()  Returns a dict mapping
+   thread id to that thread's current exception.
+*/
+PyAPI_FUNC(PyObject *) _PyThread_CurrentExceptions(void);
+
 /* Routines for advanced debuggers, requested by David Beazley.
    Don't use unless you know what you are doing! */
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Main(void);