update usage of threading.Thread.isAlive() to be is_alive() in python scripts

threading.Thread.isAlive() has been removed in python3.9. The following worker package script, src/Pegasus/tools/worker_utils.py, uses this method. Tools like pegasus-transfer will break if run on a node with python 3.9+. To fix this, usage of that method needs to be changed to is_alive(), which works in python2.7+.

References:
- https://github.com/python/cpython/pull/15225
- https://bugs.python.org/issue37804
- https://docs.python.org/2.7/library/threading.html