Improve hash map (#12678)

* Mutable _Item

* document falsy item

* resize_down: expected test result

* resize_down: actual result

This is a problem since it causes rapid cycling

* improve comment about falsy item

Co-authored-by: Andrey <Cjkjvfnby@gmail.com>

* fix long line

* Update hash_map.py

* Update hash_map.py

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Co-authored-by: Andrey <Cjkjvfnby@gmail.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>