Add new solution for the euler project problem 9 (#12771)

* Add new solution for the euler project problem 9 - precompute the squares.

* Update sol4.py

* updating DIRECTORY.md

* Update sol4.py

* Update sol4.py

* Update sol4.py

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Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>
Co-authored-by: MaximSmolskiy <MaximSmolskiy@users.noreply.github.com>