Issue 18808: blind attempt to repair some buildbot failures.

test_is_alive_after_fork is failing on some old Linux kernels, but
passing on all newer ones.  Since virtually anything can go wrong
with locks when mixing threads with fork, replace the most likely
cause with a redundant simple data member.

Lib/threading.py

@@ -552,6 +552,10 @@ class Thread:
         self._tstate_lock = None
         self._started = Event()
         self._stopped = Event()
+        # _is_stopped should be the same as _stopped.is_set().  The bizarre
+        # duplication is to allow test_is_alive_after_fork to pass on old
+        # Linux kernels.  See issue 18808.
+        self._is_stopped = False
         self._initialized = True
         # sys.stderr is not stored in the class like
         # sys.exc_info since it can be changed between instances
@@ -707,6 +711,7 @@ class Thread:
 
     def _stop(self):
         self._stopped.set()
+        self._is_stopped = True
 
     def _delete(self):
         "Remove current thread from the dict of currently running threads."
@@ -793,7 +798,7 @@ class Thread:
         assert self._initialized, "Thread.__init__() not called"
         if not self._started.is_set():
             return False
-        if not self._stopped.is_set():
+        if not self._is_stopped:
             return True
         # The Python part of the thread is done, but the C part may still be
         # waiting to run.