Force the default start method to spawn

This will no longer be necessary in python 3.14 but for now doing
this removes some deprecation warnings from the multiprocessing
tests.

tests/test_executors.py

@@ -30,6 +30,7 @@
 
 import faulthandler
 import logging
+import multiprocessing
 import os
 import signal
 import sys
@@ -704,5 +705,11 @@ def test_clobber_outputs_execute(self) -> None:
         self.assertEqual(dataset_id_1, dataset_id_3)
 
 
+def setup_module(module):
+    """Force spawn to be used if no method given explicitly."""
+    multiprocessing.set_start_method("spawn", force=True)
+
+
 if __name__ == "__main__":
+    multiprocessing.set_start_method("spawn")
     unittest.main()