Fix multiprocessing tests for Python 3.14 and newer.
Explicitly use the fork start method, since Python 3.14 changed
the default POSIX start method to forkserver. This allows tests
to launch child processes with locally defined target functions
that cannot be pickled by the forkserver start method.
--- ../tests/contrib/test_multiprocess_file_storage.py	2026-10-16 17:32:28.582513589 +0300
+++ ../tests/contrib/test_multiprocess_file_storage.py	2026-10-16 17:32:51.217840963 +0300
@@ -36,7 +36,7 @@
 def scoped_child_process(target, **kwargs):
     die_event = multiprocessing.Event()
     ready_event = multiprocessing.Event()
-    process = multiprocessing.Process(
+    process = multiprocessing.get_context('fork').Process(
         target=target, args=(die_event, ready_event), kwargs=kwargs)
     process.start()
     try: