More info from adding a faulthandler ...

    15	def f():
    16	    import ctypes
    17	
    18	for i in range(0,50):
    19	    sys.stdout.write("\r%d" % i)
    20	    sys.stdout.flush()
    21	    signal.alarm(60*2)
    22	    for j in range(0,1000):
    23	        with concurrent.futures.ProcessPoolExecutor() as executor:
    24	            ftr = executor.submit(f)
    25	            ftr.result()

Thread 0x00007f1ce7fff700 (most recent call first):
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 296 in wait
  File "/home/python/v3.7.3/lib/python3.7/multiprocessing/queues.py", line 224 in _feed
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 865 in run
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 917 in _bootstrap_inner
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 885 in _bootstrap

Thread 0x00007f1cec917700 (most recent call first):
  File "/home/python/v3.7.3/lib/python3.7/selectors.py", line 415 in select
  File "/home/python/v3.7.3/lib/python3.7/multiprocessing/connection.py", line 920 in wait
  File "/home/python/v3.7.3/lib/python3.7/concurrent/futures/process.py", line 354 in _queue_management_worker
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 865 in run
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 917 in _bootstrap_inner
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 885 in _bootstrap

Current thread 0x00007f1cfd9486c0 (most recent call first):
  File "/home/python/v3.7.3/lib/python3.7/threading.py", line 296 in wait
  File "/home/python/v3.7.3/lib/python3.7/concurrent/futures/_base.py", line 427 in result
  File "cf-deadlock.py", line 25 in <module>