Replace deprecated threading.currentThread with current_thread

[hugovk]

threading.currentThread was deprecated in Python 3.10 (October 2021).

Replace with threading.current_thread, added in Python 2.6 (October 2008).

• https://docs.python.org/3.12/whatsnew/3.10.html#deprecated
• bpo-43723: deprecate camelCase aliases from threading python/cpython#25174

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[hugovk]

I think this change is trivial enough not to require a CLA?