gh-94909: fix joining of absolute and relative Windows paths in pathlib (GH-95450)

Have pathlib use `os.path.join()` to join arguments to the `PurePath` initialiser, which fixes a minor bug when handling relative paths with drives.

Previously:

```python
>>> from pathlib import PureWindowsPath
>>> a = 'C:/a/b'
>>> b = 'C:x/y'
>>> PureWindowsPath(a, b)
PureWindowsPath('C:x/y')
```

Now:

```python
>>> PureWindowsPath(a, b)
PureWindowsPath('C:/a/b/x/y')
```