[Example 20-8][demo_executor_map.py] Race condition bug in display function

[Kristiyan-Ivanov]

On my machine, threads switch between the two print() calls in the display
function, causing output from different calls to interleave. For example, I get
multiple timestamps on the same line or mixed output from different loiter()
calls.

[22:43:48] Script starting.
[22:43:48] loiter(0): doing nothing for 0s...
[22:43:48] [22:43:48] 	loiter(1): doing nothing for 1s...loiter(0): done.

[22:43:48] 		loiter(2): doing nothing for 2s...
[22:43:48][22:43:48] 			loiter(3): doing nothing for 3s... 
results: <generator object Executor.map.<locals>.result_iterator at 0x000001E48D7C3CD0>

I tried combining the output into a single print() call, but the issue still
appears because print() with multiple arguments performs multiple writes to
stdout.

The solution I found was to use a lock.

from threading import Lock

lock = Lock()

def display(*args):
    with lock:
        print(strftime('[%H:%M:%S]'), end=' ')
        print(*args)