added doctest to playfair_cipher.py (#10823)

* added doctest to playfair_cipher.py

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

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* Added newline to EOF andremoved trailing whitespace

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

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* Apply suggestions from code review

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Co-authored-by: Keyboard-1 <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <[email protected]>