Use is_alive in favour of isAlive for Python 3.9 compatibility.

Author: tirkarthi

packages/mbed-greentea/mbed_greentea/mbed_greentea_cli.py

@@ -902,7 +902,7 @@ def get_parallel_value(value):
                 # A time of 0.1 seconds is a fairly arbitrary choice. It needs
                 # to balance CPU utilization and responsiveness to keyboard interrupts.
                 # Checking 10 times a second seems to be stable and responsive.
-                while t.isAlive():
+                while t.is_alive():
                     t.join(0.1)
 
                 test_return_data = test_result_queue.get(False)