Path Conversion Issue in pathname2url Impacts Browser Launch on mingw-w64

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Description

When using Jupyter Server on mingw-w64 Python builds, the server fails to open the browser automatically. This issue is traced back to urllib.request.pathname2url, which produces malformed URLs when handling Windows paths with forward slashes. The malformed URLs (e.g., '///C://Users/...) prevent browsers from recognizing the paths correctly.

Steps to Reproduce

1. Install mingw-w64-python-jupyter_server (via MSYS2).
2. Start Jupyter Lab or Server with any typical command:

   jupyter lab

3. Observe that the browser does not open, and the following URL is printed:

   file:///C://Users/.../runtime/jpserver-XXXXX-open.html
   

4. Manually copy the URL into a browser and notice that it fails to open.
5. Compare with the expected URL:

   file://C:/Users/.../runtime/jpserver-XXXXX-open.html
   

Analysis

jupyter_server/jupyter_server/utils.py

Line 28 in e544fa1

from urllib.request import pathname2url as _pathname2url

jupyter_server/jupyter_server/utils.py

Line 39 in e544fa1

pathname2url = _pathname2url

jupyter_server/jupyter_server/serverapp.py

Line 3019 in e544fa1

assembled_url = urljoin("file:", pathname2url(open_file))

Also reported on msys2-contrib/cpython-mingw#182

The root cause appears to be a bug in urllib.request.pathname2url, which mingw-w64 Python inherits. When converting paths, it doubles the forward slashes after the drive letter (C://) instead of normalizing to the expected format.

from urllib.request import pathname2url

print(pathname2url("C:/Users/example"))
# Output: '///C://Users/example' (Incorrect)
# Expected: '//C:/Users/example'

Expected Behavior

pathname2url should produce URLs in the format:

file://C:/Users/.../runtime/jpserver-XXXXX-open.html

This would allow browsers to open correctly, resolving the issue.

Context

• Operating System: Windows (mingw-w64 environment, MSYS2)
• Browser: Any modern browser
• Jupyter Server version: 2.14.2-2
• Python version: mingw-w64 Python (3.12.7)