Terminate netbox client threadpool on exit

[azryve]

From the mp.ThreadPool documentation.

resources must also be properly managed, either by using the pool as a context manager or by calling close() and terminate() manually.

Without explicit close() or terminate() it can produce exceptions on exit when __del__ is called, before the fds were opened:

Exception ignored while calling deallocator <function Pool.__del__ at 0x10a340bf0>: Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/multiprocessing/pool.py", line 271, in __del__
    self._change_notifier.put(None)
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/multiprocessing/queues.py", line 397, in put
    self._writer.send_bytes(obj)
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/multiprocessing/connection.py", line 206, in send_bytes
    self._send_bytes(m[offset:offset + size])
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/multiprocessing/connection.py", line 444, in _send_bytes
    self._send(header + buf)
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/multiprocessing/connection.py", line 400, in _send
    n = write(self._handle, buf)
OSError: [Errno 9] Bad file descriptor

Much cleaner would be to a use context manager but BaseNetboxClient does not provide one, instead doing init_pool in the constructor. This leaves atexit as a least involved option to prevent this annoying traceback on the shutdown.