Faster implementation of neighbors_set function.

We use Python sets and avoid the unnecessary creation of new sets.

Old version:
sage: %time neighbors_set(graphs.PetersenGraph(), {1,3,5})
CPU times: user 1.01 ms, sys: 125 µs, total: 1.13 ms
Wall time: 1.08 ms
[0, 2, 4, 6, 7, 8]

New version:
sage: %time neighbors_set(graphs.PetersenGraph(), {1,3,5})
CPU times: user 489 µs, sys: 46 µs, total: 535 µs
Wall time: 542 µs
[0, 2, 4, 6, 7, 8]