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

Author: cxzhong

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):