`Future.__del__` logs ignored exceptions during interpreter shutdown

While I don't have a reproducer, multiple users have reported seeing many logs about `Future.__del__` ignoring exceptions during interpreter shutdown. #8449 was supposed to address this, but it failed to do so:

Before #8449

```python3
Exception ignored in:
Traceback (most recent call last):
File "/python/lib/python3.10/site-packages/distributed/client.py", line 510, in __del__
AttributeError: 'NoneType' object has no attribute 'is_finalizing'
```

After #8449
```python3
Exception ignored in:
Traceback (most recent call last):
File "/python/lib/python3.10/site-packages/distributed/client.py", line 510, in __del__
TypeError: 'NoneType' object is not callable
```

From what I understand, this fails because of this caveat: https://github.com/python/cpython/blob/e39ae6bef2c357a88e232dcab2e4b4c0f367544b/Doc/c-api/init.rst#L403-L405

Empirically, we can fix the `Future.__del__` problem by binding `sys.is_finalizing` to a class variable which will avoid its destruction. However, I assume that `is_python_shutting_down` has a similar problem. While this issue isn't urgent, the chattiness is annoying and might distract from genuine issues on shutdown.