Make get_random_approval_profile deterministic

[mdbrnowski]

Before this change, the code below gives different outcomes (despite random.seed()) because set (and thus Instance) is unordered. After the change, it always produces the same result.

import random
from pabutools.election.instance import get_random_instance
from pabutools.election.profile import get_random_approval_profile

random.seed(42)
instance = get_random_instance(3, 1, 10)
profile = get_random_approval_profile(instance, 10)
print(profile)

One could instead add sorted() in get_random_approval_ballot(), but this could negatively impact performance.
Therefore, I decided to leave get_random_approval_ballot() non-deterministic by default (unless, of course, the user passes an ordered Iterable as an argument).