Create a function which returns the type of triangle, given the side lengths. Return the following values if they match the criteria.
"scalene""isosceles""equilateral""not a triangle"get_triangle_type([2, 6, 5]) ➞ "scalene"
get_triangle_type([4, 4, 7]) ➞ "isosceles"
get_triangle_type([8, 8, 8]) ➞ "equilateral"
get_triangle_type([3, 5, 5, 2]) ➞ "not a triangle"