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Unconnected Vertices
In LockedIn, it's sad, but there are users who have zero connections.
Womp womp.
Your task is to find these unconnected vertices in the graph and return them as a list.
Assignment
Complete the unconnected_vertices(self) method. It should return a list of vertices (integers) that have no connections. A vertex with no edges will have an empty set as its value in the self.graph dictionary.