I would expect the following to work:
```
>>> import pathlib
>>> pathlib.Path.cwd().parents[0:1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...\Python36\lib\pathlib.py", line 593, in __getitem__
    if idx < 0 or idx >= len(self):
TypeError: '<' not supported between instances of 'slice' and 'int'
```

Since pathlib documents `parents` as a sequence-type, and slicing a sequence is pretty standard behavior.

If it is deemed a bug which needs to be fixed, I've gone ahead and attached the PR to fix it.

CLA signage is pending approval at the company I work for, with most people out for the holidays (so it might be a day or two turnaround).

See also issue21041. First than add support for slices, we should make a decision about negative indices.


In any case this is a new feature, which can be only added in the future 3.8 release.