Add type hints and improve generate_parentheses_iterative (#14324)

* Add type hints and improve code quality for generate_parentheses_iterative

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Fix line length issue

* Update generate_parentheses_iterative.py

* Update generate_parentheses_iterative.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>