Enhance readability of N Queens

[Hardvan]

Enhanced readability and added comments to the n_queens.py code. Replaced repeated len(board) with a single variable n for efficiency in the is_safe function.

• Fix a bug or typo in an existing algorithm?
• Add an algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

Additional Information:

This pull request improves the readability of the n_queens.py code and optimizes the is_safe function by replacing repeated calls to len(board) with a single variable n. These changes make the code more understandable and efficient.