Fix multiprocessing start method for Python 3.14 compatibility

[cxzhong]

Python 3.14 changed the default multiprocessing start method from fork to forkserver on Linux. This caused pickling errors in the doctest framework because DocTestWorker objects contained unpicklable local functions.

The fix ensures that the fork start method is always used on all platforms, not just macOS, since Sage's doctesting framework requires the 'fork' method to function correctly.

Changes:

• src/sage/doctest/forker.py: Remove Darwin-only check, always use fork
• src/sage/doctest/external.py: Remove Darwin-only check, always use fork

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[Copilot]

Pull Request Overview

This PR fixes multiprocessing compatibility issues with Python 3.14 by ensuring Sage's doctest framework always uses the 'fork' start method. Python 3.14 changed the default multiprocessing start method from 'fork' to 'forkserver' on Linux, which causes pickling errors in Sage's doctesting framework since DocTestWorker objects contain unpicklable local functions.

• Removes platform-specific (Darwin-only) checks for setting the multiprocessing start method
• Updates comments to explain the Python 3.14 compatibility requirement
• Ensures 'fork' method is used consistently across all platforms

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/sage/doctest/forker.py	Removes Darwin-only check and always sets 'fork' method with updated comment
src/sage/doctest/external.py	Removes Darwin-only check and always sets 'fork' method with updated comment

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[cxzhong]

@dimpase I think we should move on this. without this, the doctest goes wrong in python 3.14.

[dimpase]

OK

[dimpase]

this should be conditional on the Python version. What's OK for 3.14 is not good for <3.14

[cxzhong]

this should be conditional on the Python version. What's OK for 3.14 is not good for <3.14

But seems all CI pass under python 3.13 and older versions. and before python 3.14, the default start method of multiprocess is fork on linux and spawn on MacOS. But in python 3.14+, It has changed to forkserver on linux. you can refer to https://docs.python.org/it/3.14/whatsnew/3.14.html

On Unix platforms other than macOS, “forkserver” is now the the default start method for ProcessPoolExecutor (replacing “fork”). This change does not affect Windows or macOS, where “spawn” remains the default start method.

If the threading incompatible fork method is required, you must explicitly request it by supplying a multiprocessing context mp_context to ProcessPoolExecutor.

See forkserver restrictions for information and differences with the fork method and how this change may affect existing code with mutable global shared variables and/or shared objects that can not be automatically pickled.

Besides, our forker.py and externel.py must rely on fork.

[dimpase]

Right, I misread your patch. apologies. All good.