Update the comparison implementation in voronoi.py (fixes #18219)

[havatv]

Description

New (after second commit): The cmp function introduced with the Python 2 -> 3 upgrade contained a greater than (>), but the gt function was missing in Site and Halfedge. I have changed cmp to only use less than (<), and have modified the lt functions to return False in the last else.
Old: When "translating" the cmp function of Site and Halfedge to Python 3, the gt function was forgotten. It has been added.

Checklist

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[nyalldawson]

Thanks for your hard work here!

I'd love to get a unit test in place protecting this situation. Are you comfortable adding one, or would you like some guidance?

[havatv]

I have no experience with unit tests, but I could provide a dataset that covers more cases than the existing one. Several datasets would be needed to cover more cases.
Is it enough to add input and result datasets and updating qgis_algorithm_tests.yaml?
And sorry for changing so much in the latest commit - I found out that the gt function could be avoided by changing the cmp function to only use < (less than). The combined changes of these commits have now become very small.

[havatv]

I tried to add a test in pull request #7557.

[nyalldawson]

Thank you!