Support __index__() in getrandbits(), randbytes() and others

Feature or enhancement

Support of the __index__() method in getrandbits() and randbytes() is inconsistent.

• Random.getrandbits() supports __index__(), but SystemRandom.getrandbits() does not.
• SystemRandom.randbytes() supports __index__(), but Random.randbytes() does not.

For consistence, both classes should support it.

sample() fails if its k argument is not integer. k should have __index__(), but it also should support comparison with integers and multiplication by list and integer. It is easy to make it more general, and only require __index__(). It will also produce more meaningful error.

binomialvariate() seems "working" with non-integer n argument, but the result is questionable. Passing non-integer value is most likely an error, and it would be better to detect it.

cc @rhettinger

Individua le implementazioni e i test di Random.getrandbits(), SystemRandom.getrandbits(), Random.randbytes(), SystemRandom.randbytes(), sample() e binomialvariate(). Confronta la gestione degli argomenti non interi, quindi verifica che gli argomenti supportati utilizzino coerentemente index() e che gli input non validi di binomialvariate() vengano rifiutati.