ntpath not adding slash after drive letters

# Bug report

### Bug description:

```python
import ntpath
print(ntpath.join('c:', 'x', 'y'))
```

I would expect the following code to print `'c:\\x\\y'`, yet it prints `'c:x\\y'`, Would you expect a PR that changes the current behaviour? 


### CPython versions tested on:

3.10, 3.11, 3.12, 3.13, CPython main branch

### Operating systems tested on:

Linux, Windows