Fix random neighbors selection for Python >= 3.9

[Alexyyym]

In node classes that implement a random selection between their neighbors (for example EL), the method is defined as follows:

def get_neighbors(self, node=None):
    return set(self.rng.sample(self.my_neighbors, self.degree))

With:

• self.rng being the Random library.
• self.my_neighbors being a set of neighbors.

For Python >=3.9, the method Random.sample() is deprecated for sets. According to the documentation, "In the future, the population must be a sequence. Instances of set are no longer supported. The set must first be converted to a list or tuple, preferably in a deterministic order so that the sample is reproducible."