Use is_alive instead of isAlive for Python 3.9 compatibility. (celery#5898)

Author: tirkarthi

celery/utils/timer2.py

@@ -102,7 +102,7 @@ def stop(self):
             self.running = False
 
     def ensure_started(self):
-        if not self.running and not self.isAlive():
+        if not self.running and not self.is_alive():
             if self.on_start:
                 self.on_start(self)
             self.start()