bpo-35283: Add pending deprecation warning for Thread.isAlive (GH-11604)

Add a pending deprecated warning for the threading.Thread.isAlive() method.
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 8160a5a..27f328d 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -415,7 +415,8 @@
         t.setDaemon(True)
         t.getName()
         t.setName("name")
-        t.isAlive()
+        with self.assertWarnsRegex(PendingDeprecationWarning, 'use is_alive()'):
+            t.isAlive()
         e = threading.Event()
         e.isSet()
         threading.activeCount()