Update Conditional Logic for Grade Assignment

[Shivay-Shakti]

Summary

This pull request updates the conditional logic used for assigning letter grades based on numerical scores.

Changes

• Changed the condition for a 'C' grade to check for scores between 70 and 79 inclusive.
• Updated the condition for a 'B' grade to check for scores between 80 and 89 inclusive.

Motivation

The previous logic incorrectly assigned a 'C' grade to any score above 70 and a 'B' grade for any score above 80, without upper bounds. This update corrects the grade assignment to reflect the standard grading scale.

Testing

• Added test cases for the boundary conditions to ensure the grades are assigned correctly.
• Ran the entire test suite to confirm that no existing functionality is broken.

[github-actions]

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[alee]

Thanks for taking the time to submit this PR! @sborrego submit the PR right before this that also addresses this issue in a simpler way with less conditional checks so I think we will go with that one. I'm guessing you all caught this during a carpentries workshop?

Thanks again, we appreciate your contributions!

[Shivay-Shakti]

That's amazing, yes we found it during the workshop. Thanks, @sborrego and @alee for your pro-activeness!