gh-41023: Fix multiprocessing start method for Python 3.14 compatibility
    
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

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URL: #41023
Reported by: Chenxin Zhong
Reviewer(s): Copilot, Dima Pasechnik

Author: Release Manager

src/sage/doctest/external.py

@@ -37,8 +37,9 @@
 # With OS X, Python 3.8 defaults to use 'spawn' instead of 'fork' in
 # multiprocessing, and Sage doctesting doesn't work with 'spawn'. See
 # trac #27754.
-if platform.system() == 'Darwin':
-    multiprocessing.set_start_method('fork', force=True)
+# With Python 3.14, the default changed to 'forkserver' on Linux as well.
+# Sage doctesting requires 'fork' method.
+multiprocessing.set_start_method('fork', force=True)
 Array = multiprocessing.Array
 
 # Functions in this module whose name is of the form 'has_xxx' tests if the

src/sage/doctest/forker.py

@@ -91,8 +91,9 @@
 # With OS X, Python 3.8 defaults to use 'spawn' instead of 'fork' in
 # multiprocessing, and Sage doctesting doesn't work with 'spawn'. See
 # trac #27754.
-if platform.system() == 'Darwin':
-    multiprocessing.set_start_method('fork', force=True)
+# With Python 3.14, the default changed to 'forkserver' on Linux as well.
+# Sage doctesting requires 'fork' method.
+multiprocessing.set_start_method('fork', force=True)
 
 
 def _sorted_dict_pprinter_factory(start, end):