pathlib.Path('/').mkdir() raises an IsADirectoryError instead of a FileExistsError.

It looks like this is an OSX-specific behavior, and not a python problem:
 $ mkdir .
mkdir: .: File exists
 $ mkdir /
mkdir: /: Is a directory

The mkdir method needs a fix similar to what was done for issue 25583. For example, currently on Windows the exist_ok option doesn't handle the PermissionError raised when [accidentally] trying to create the root directory:

    >>> pathlib.Path('C:/').mkdir(exist_ok=True)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "C:\Program Files\Python 3.5\lib\pathlib.py", line 1199, in mkdir
        self._accessor.mkdir(self, mode)
      File "C:\Program Files\Python 3.5\lib\pathlib.py", line 371, in wrapped
        return strfunc(str(pathobj), *args)
    PermissionError: [WinError 5] Access is denied: 'C:\\'

I can't test on Windows and OSX but I suppose PR 805 fixes both issues.

New changeset af7b9ec5c855366feef4c67dc492d64b3baf84ca by Serhiy Storchaka in branch 'master':
bpo-25803: Avoid incorrect errors raised by Path.mkdir(exist_ok=True) (#805)
https://github.com/python/cpython/commit/af7b9ec5c855366feef4c67dc492d64b3baf84ca

New changeset 8c8785b2f8e4048cef350f89c686266f4519b67c by Serhiy Storchaka in branch '3.6':
bpo-25803: Avoid incorrect errors raised by Path.mkdir(exist_ok=True)… (#806)
https://github.com/python/cpython/commit/8c8785b2f8e4048cef350f89c686266f4519b67c

New changeset 1fc1f8d7f737f80a1ee5ea37b9781c0a790102b2 by Serhiy Storchaka in branch '3.5':
bpo-25803: Avoid incorrect errors raised by Path.mkdir(exist_ok=True) (#805) (#807)
https://github.com/python/cpython/commit/1fc1f8d7f737f80a1ee5ea37b9781c0a790102b2