Fix NotebookApp startup error on Windows
Fix NotebookApp startup error on Windows

Convert all backslashes into forward slashes. This fixes the following startup exception, due to mixing forward and back slashes:
```
[E 10:34:29.657 NotebookApp] Exception while loading config file C:\Users\sftnight\.rootnb\jupyter_notebook_config.py
    Traceback (most recent call last):
      File "C:\Python38-32\lib\site-packages\traitlets\config\application.py", line 738, in _load_config_files
        config = loader.load_config()
      File "C:\Python38-32\lib\site-packages\traitlets\config\loader.py", line 614, in load_config
        self._read_file_as_dict()
      File "C:\Python38-32\lib\site-packages\traitlets\config\loader.py", line 646, in _read_file_as_dict
        exec(compile(f.read(), conf_filename, 'exec'), namespace, namespace)
      File "C:\Users\sftnight\.rootnb\jupyter_notebook_config.py", line 6
        c.NotebookApp.extra_static_paths = ['C:/Users/sftnight/build/release\js\']
```

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