Fix incorrect __getitem__ method in Path

[qoo2p5]

Plumbum doesn't support PathLike logics, because Path class is inherited from str and therefore considered as str in python libraries (__fspath__ method is never called). Before #455 everything was fine, but after the corresponding commit libraries are unable to use plumbum Path as string, because __getitem__ returns strange thinks for int and slice keys.

Consider the following example:

import os
import plumbum

print(os.path.splitext(plumbum.local.path('/test/file.ext')))

Before #455 the output is as expected:

('/test/file', '.ext')

But after the commit it is meaningless:

(<LocalPath /test/file.ext/slice(None, 10, None)>, <LocalPath /test/file.ext/slice(10, None, None)>)

I fix this bug by returning the old __getitem__ behaviour for int and slice keys.

[qoo2p5]

Also, I think a new version must be released, because this bug is really critical.

[henryiii]

Thanks!