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

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