zerver/lib/timeout.py: Replace isAlive by is_alive.

In threading.Thread, isAlive has been removed in python 3.
We should use is_alive instead.

tools/run-mypy

@@ -51,7 +51,6 @@ zerver/lib/email_mirror.py
 zerver/lib/notifications.py
 zerver/lib/parallel.py
 zerver/lib/test_helpers.py
-zerver/lib/timeout.py
 zerver/lib/tornado_ioloop_logging.py
 zerver/management/commands/create_stream.py
 zerver/management/commands/email-mirror.py

zerver/lib/timeout.py

@@ -74,7 +74,7 @@ def timeout(timeout, func, *args, **kwargs):
     thread.start()
     thread.join(timeout)
 
-    if thread.isAlive():
+    if thread.is_alive():
         # Gamely try to kill the thread, following the dodgy approach from
         # http://stackoverflow.com/a/325528/90777
         #
@@ -83,7 +83,7 @@ def timeout(timeout, func, *args, **kwargs):
         for i in range(10):
             thread.raise_async_timeout()
             time.sleep(0.1)
-            if not thread.isAlive():
+            if not thread.is_alive():
                 break
         raise TimeoutExpired