Have gdb.ThreadExitedEvent inherit from gdb.ThreadEvent

The documentation says that ThreadExitedEvent is derived from
ThreadEvent, but the code does not actually implement this.

This patch fixes the problem.  I propose applying this to gdb 17 as
well.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33444
Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/python/py-event-types.def

@@ -54,7 +54,7 @@ GDB_PY_DEFINE_EVENT_TYPE (new_thread,
 GDB_PY_DEFINE_EVENT_TYPE (thread_exited,
 			  "ThreadExitedEvent",
 			  "GDB thread exited event object",
-			  event_object_type);
+			  thread_event_object_type);
 
 GDB_PY_DEFINE_EVENT_TYPE (new_inferior,
 			  "NewInferiorEvent",

gdb/testsuite/gdb.python/py-thread-exited.py

@@ -26,6 +26,8 @@ def thread_exited_handler(event):
     global threadOneExit, threadTwoExit, mainThreadExit
     print("{}".format(event))
     assert isinstance(event, gdb.ThreadExitedEvent)
+    # Also check the inheritance.
+    assert isinstance(event, gdb.ThreadEvent)
     if threadOneExit == "":
         threadOneExit = "event type: thread-exited. global num: {}".format(
             event.inferior_thread.global_num