Add factorial_iterative implementation in Python

[agarwalpranay02]

This PR adds an iterative factorial implementation at maths/factorial_iterative.py.
Includes input validation and a small demonstration in main. Beginner-friendly contribution for Hacktoberfest.

[algorithms-keeper]

Closing this pull request as invalid

@agarwalpranay02, this pull request is being closed as none of the checkboxes have been marked. It is important that you go through the checklist and mark the ones relevant to this pull request. Please read the Contributing guidelines.

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[agarwalpranay02]

Hello maintainers — this is a beginner contribution for Hacktoberfest. Please review when you have a moment. Thank you!