un-break Python 2.5 compatibility by using isAlive() instead of is_alive()

Python's documentation has a bug[1], in that it doesn't correctly annotate
is_alive as being a function introduced in Python 2.6.

[1] http://bugs.python.org/issue15126

Signed-off-by: Steven Noonan <snoonan@amazon.com>

paramiko/transport.py

@@ -1389,7 +1389,7 @@ class Transport (threading.Thread):
     def stop_thread(self):
         self.active = False
         self.packetizer.close()
-        while self.is_alive():
+        while self.isAlive():
             self.join(10)