Added function that checks if a string is an isogram (#7608)

* Added function that checks if a string is an isogram.

* Added wiki reference and fixed comments.

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

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* Made function name more self-documenting. Raise ValueError if string contains 1 or more digits. Renamed file. Lowercase string inside function.

* Removed check_isogram.py (file renamed).

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

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

* Fixed test failure.

* Raise ValueError when string has non-alpha characters. Removed import.

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