`pathlib.PurePath.parents` is a sequence [1] but it rejects negative indexes:

  >>> from pathlib import PurePath
  >>> PurePath('a/b/c').parents[-2]
  Traceback (most recent call last):
  ...
  IndexError: -2

Sequences in Python interpret negative indexes as `len(seq) + i` [2]

I've included the patch that fixes the issue and adds corresponding tests. No documentation changes are needed.

[1]: http://docs.python.org/3/library/pathlib#pathlib.PurePath.parents
[2]: http://docs.python.org/3/library/stdtypes.html#sequence-types-list-tuple-range