Bug in xjoin with Windows pathnames
{'login': 'albertvillanova', 'id': 8515462, 'node_id': 'MDQ6VXNlcjg1MTU0NjI=', 'avatar_url': 'https://avatars.githubusercontent.com/u/8515462?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/albertvillanova', 'html_url': 'https://github.com/albertvillanova', 'followers_url': 'https://api.github.com/users/albertvillanova/followers', 'following_url': 'https://api.github.com/users/albertvillanova/following{/other_user}', 'gists_url': 'https://api.github.com/users/albertvillanova/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/albertvillanova/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/albertvillanova/subscriptions', 'organizations_url': 'https://api.github.com/users/albertvillanova/orgs', 'repos_url': 'https://api.github.com/users/albertvillanova/repos', 'events_url': 'https://api.github.com/users/albertvillanova/events{/privacy}', 'received_events_url': 'https://api.github.com/users/albertvillanova/received_events', 'type': 'User', 'site_admin': False}
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{'login': 'albertvillanova', 'id': 8515462, 'node_id': 'MDQ6VXNlcjg1MTU0NjI=', 'avatar_url': 'https://avatars.githubusercontent.com/u/8515462?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/albertvillanova', 'html_url': 'https://github.com/albertvillanova', 'followers_url': 'https://api.github.com/users/albertvillanova/followers', 'following_url': 'https://api.github.com/users/albertvillanova/following{/other_user}', 'gists_url': 'https://api.github.com/users/albertvillanova/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/albertvillanova/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/albertvillanova/subscriptions', 'organizations_url': 'https://api.github.com/users/albertvillanova/orgs', 'repos_url': 'https://api.github.com/users/albertvillanova/repos', 'events_url': 'https://api.github.com/users/albertvillanova/events{/privacy}', 'received_events_url': 'https://api.github.com/users/albertvillanova/received_events', 'type': 'User', 'site_admin': False}
[{'login': 'albertvillanova', 'id': 8515462, 'node_id': 'MDQ6VXNlcjg1MTU0NjI=', 'avatar_url': 'https://avatars.githubusercontent.com/u/8515462?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/albertvillanova', 'html_url': 'https://github.com/albertvillanova', 'followers_url': 'https://api.github.com/users/albertvillanova/followers', 'following_url': 'https://api.github.com/users/albertvillanova/following{/other_user}', 'gists_url': 'https://api.github.com/users/albertvillanova/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/albertvillanova/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/albertvillanova/subscriptions', 'organizations_url': 'https://api.github.com/users/albertvillanova/orgs', 'repos_url': 'https://api.github.com/users/albertvillanova/repos', 'events_url': 'https://api.github.com/users/albertvillanova/events{/privacy}', 'received_events_url': 'https://api.github.com/users/albertvillanova/received_events', 'type': 'User', 'site_admin': False}]
Currently, `xjoin` function has a bug with local Windows pathnames: instead of returning the OS-dependent join pathname, it always returns it in POSIX format.

```python
from datasets.download.streaming_download_manager import xjoin

path = xjoin("C:\\Users\\USERNAME", "filename.txt")
```

Join path should be:
```python
"C:\\Users\\USERNAME\\filename.txt"
```
However it is:
```python
"C:/Users/USERNAME/filename.txt"
```
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