Fix Python 3.14 compatibility: Set multiprocessing start method to 'fork'

Python 3.14 changed the default multiprocessing start method from 'fork'
to 'forkserver' on Linux, which breaks SIGCHLD signal delivery in tests.
This causes pselect tests to hang indefinitely.

Changes:
- Add Python 3.14 detection in conftest.py
- Force multiprocessing to use 'fork' method for Python 3.14+
- Add defensive check for __test__ attribute before deletion

Fixes test timeouts on Python 3.14.0rc2.
All tests now pass in ~8 seconds (previously timed out at 300s+).

Author: cxzhong

src/conftest.py

@@ -1,5 +1,6 @@
 import pathlib
 import platform
+import sys
 
 from _pytest.nodes import Collector
 from _pytest.doctest import DoctestModule
@@ -18,6 +19,16 @@
         "cysignals/tests.pyx",
     ]
 
+# Python 3.14+ changed the default multiprocessing start method to 'forkserver'
+# on Linux, which breaks SIGCHLD-based tests. Set it back to 'fork' for compatibility.
+if sys.version_info >= (3, 14) and platform.system() != "Windows":
+    import multiprocessing
+    try:
+        multiprocessing.set_start_method('fork', force=True)
+    except RuntimeError:
+        # Method may already be set
+        pass
+
 
 def pytest_collect_file(
     file_path: pathlib.Path,
@@ -31,6 +42,7 @@ def pytest_collect_file(
             _, module_name = resolve_pkg_root_and_module_name(file_path)
             module = importlib.import_module(module_name)
             # delete __test__ injected by cython, to avoid duplicate tests
-            del module.__test__
+            if hasattr(module, '__test__'):
+                del module.__test__
             return DoctestModule.from_parent(parent, path=file_path)
     return None