Handle `/:/` for `ntpath.isabs`

# Bug report

### Bug description:

```python
>>> import ntpath
>>> ntpath.isabs("/:/foo")
True
```

Expected: `False`, as `/:/foo` is relative to the drive or UNC share of the current working directory.

### CPython versions tested on:

CPython main branch

### Operating systems tested on:

Windows


### Linked PRs
* gh-117362