Summary

This PR addresses an issue I spotted on #145300, which mentioned that itertools.islice does not support __length_hint__ currently.

General philosophy is

• Early return 0 in all cases that make sense
• If stop is not set, we don't know the number of elements we will iterate over, so we use the underlying iterator's __length_hint__ then do our calculations over that
• Once we have a start, a stop and a step, or a start a length hint and a step, we can know how many iterations that will result in.

Testing wise, I added a few unit tests but it's unclear to me if I should be adding more (there doesn't seem to be a ton of instances/examples for the other itertools tests), but feel free to request more. Just to gain confidence in the change myself, I also tried the following locally (it works):

import itertools

for start in range(0, 100):
    for stop in range(0, 100):
        for step in range(1, 100):
            r = range(100)
            s = itertools.islice(r, start, stop, step)
            hint = s.__length_hint__()
            l = list(s)

            assert len(l) == hint

Note this is my first PR here; I read the contributor guidelines but I apologise in advance if anything I did not follow them (please do tell me!)

• Issue: Inconsistent .extend() behavior in bytearray #145300