Python 3.9 introduced a regression with concurrent.futures. The following program works fine on Python < 3.8 but raises an error on 3.9:

```
import atexit
import concurrent.futures

def spawn():
    with concurrent.futures.ThreadPoolExecutor() as t:
        pass

atexit.register(spawn)
```

```
$ python3.9 rep.py
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/usr/local/Cellar/python@3.9/3.9.0_3/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/thread.py", line 37, in <module>
    threading._register_atexit(_python_exit)
  File "/usr/local/Cellar/python@3.9/3.9.0_3/Frameworks/Python.framework/Versions/3.9/lib/python3.9/threading.py", line 1370, in _register_atexit
    raise RuntimeError("can't register atexit after shutdown")
RuntimeError: can't register atexit after shutdown
```