__del__ beside the __dealloc__

[brightening-eyes]

python3 uses del for uninitialization of classes, it would be good to have this as well, it can help to uninitialize python variables, while in dealloc we can uninitialize c variables.
this is useful for example when we are using a module that uses ctypes, or is dependent on some python initialization.

[scoder]

Right, PEP 442 allows this. Cython currently implements it the other way round and calls tp_finalize() from tp_dealloc(), but it would certainly be nice to also support __del__ directly. PR welcome.

How to do it:
The tp_finalize slot in TypeSlots.py needs to become a MethodSlot backed by __del__.
This seems worth a dedicated set of tests, maybe tests/run/pep442_tp_finalize.pyx.