Python extension leaks the thread hook callbacks

[pepone]

I think this objects are leaked, the ThreadHook increase the ref count but never decreases it. If that is on purpose we should add a comment.

https://github.com/zeroc-ice/ice/blob/f04017972bf5913dd0b32ebcd4126cffa7b955de/python/modules/IcePy/Thread.cpp#L54-L56