`islice` has different behavior from the standard `itertools` version.

[kbsriram]

While writing tests for a different issue, noticed a few differences with the current islice implementation.

Some examples:

>>> import adafruit_itertools as ait
>>> import itertools as it
>>> list(it.islice("ab", 10))
['a', 'b']
>>> list(ait.islice("ab", 10))
Traceback (most recent call last):
  File "/home/kbs/venv/lib/python3.8/site-packages/adafruit_itertools/__init__.py", line 353, in islice
    yield next(it)
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: generator raised StopIteration
>>> list(it.islice("ab", -1))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Stop argument for islice() must be None or an integer: 0 <= x <= sys.maxsize.
>>> list(ait.islice("ab", -1))
[]
>>> 

I can follow-up with a fix to more closely align the behavior, if this feels worth fixing?

[dhalbert]

Certainly, PR's are welcome to align it better with the CPython version.