stdin-filename doesn't support UNC path in python 3.8/3.9 on Windows

[guotuofeng]

Describe the bug

In Windows, the UNC path is not supported when using as --stdin-filename. For example, let's say one path is: \\?\D:\git\OLive\setup.py

When running

python.exe -mblack --stdin-filename \\?\D:\git\OLive\setup.py -v -

The following error message will be shown.

To Reproduce

Windows Python pathlib doesn't support UNC path.
Expected behavior
UNC path \\?\D:\git\OLive\setup.py should be supported in Windows allow as stdin-filename

Environment

• Windows
• Python 3.8.18
• black: 24.1.1

• Black's version:
• OS and Python version:

Additional context