Use itertools.product() instead of nested for-loops

[vterron]

This is arguably more readable, and introduces students to the always amazing itertools module.

[astrojuanlu]

Thanks @vterron! I'd rather disagree that this is more readable than the double for loop though. From a novice point of view, this introduces a new import and a new function that one has to explain. If you rephrase this PR leaving that cell as-is and introducing a new one explaining the new syntax I will merge it :)