os.fspath() returns its argument if it is a str. That means that it bypasses __fspath__ for str subclasses.

This is the case for the library path.py for example.

This is a corner case that was discovered while trying to fix https://github.com/matplotlib/matplotlib/issues/11306

Minimal example:

```
import os

class MyPath(str):
    def __fspath__(self):
        print("Returns a pure string")
        return str(self)

os.fspath(MyPath())  # Prints nothing
```