Avoid stacktrace on process exit in Client.__del__()

[noirbee]

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Description of change

Client.close() may call ConnectionPool.release() or ConnectionPool.disconnect(); both methods may end up calling os.getpid() (through ConnectionPool._checkpid() or threading.Lock() (through ConnectionPool.reset()). As mentioned in the Python documentation [1], at interpreter shutdown, module globals (in this case, the os and threading module references) may be deleted or set to None before del() methods are called. This causes an AttributeError to be raised when trying to run e.g. os.getpid(); while the error is ignored by the interpreter, the traceback is still printed out to stderr.

Closes #3014

[1] https://docs.python.org/3/reference/datamodel.html#object.__del__

[noirbee]

Note that I unfortunately cannot provide an easy way to reproduce the issue; I can only reproduce it using our own codebase. I suspect this is due to the fact that we load a much larger amount of code than what a synthetic / minimal reproduction attempt would.