Change some os.path stubs to acknowledge __fspath__ protocol support in >=Python3.6

[brycepg]

Example:

from pathlib import Path
import os
os.path.join(Path("."), '..')

Is valid python because Python3.6 os.path has been changed to accept the __fspath__ protocol
However mypy outputs:

t.py:3: error: Value of type variable "AnyStr" of "join" cannot be "object"

Relevant location: https://github.com/python/typeshed/blob/master/stdlib/3/os/path.pyi#L79