While in theory it would be good to have calls to super classes'
`__del__()` destructors in subclasses, matching the ones to
`__init__()`, several limitations of current CPython implementation do
not make it a practical requirement.
So remove `__del__` from examples, and add a note summarizing the
current problems with using it (aka `tp_finalize` in C++ code).
Also see !132476 for some discussion about that topic.
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