fix: Don't use mp.get_context("spawn") in test (#3753)

This is deprecated it seems.

Examples/Python/tests/test_reader.py

@@ -290,8 +290,11 @@ def test_edm4hep_simhit_particle_reader(tmp_path):
     tmp_file = str(tmp_path / "output_edm4hep.root")
     odd_xml_file = str(getOpenDataDetectorDirectory() / "xml" / "OpenDataDetector.xml")
 
-    with multiprocessing.get_context("spawn").Pool() as pool:
-        pool.apply(generate_input_test_edm4hep_simhit_reader, (odd_xml_file, tmp_file))
+    p = multiprocessing.Process(
+        target=generate_input_test_edm4hep_simhit_reader, args=(odd_xml_file, tmp_file)
+    )
+    p.start()
+    p.join()
 
     assert os.path.exists(tmp_file)