Minor text correction.

This is just a minor grammatical error, but since it tripped me up for a second hopefully it's helpful to point out: the third "self-test" assertion has the comment

"It's can not be a triangle"

The comment is reminding users that the inputs may not form a valid triangle. Some better text might be

"It can be not a triangle."

This is because in English moving the "not" before the verb ("be") makes the sentence an imperative, saying that the object of the sentence "It" is not allowed to be a triangle.

Hopefully this is helpful and permitted and not just nitpicking.